Creating spin-transfer torque in oscillators and memories

ABSTRACT

A structure includes an electrically conductive material possessing spontaneous magnetization (“free magnet”) not in contact with an electrically resistive material possessing spontaneous magnetization (“pinned magnet”), and a spacer having free electrons to transfer spin between the electrically resistive material and the electrically conductive material. During operation, an existing direction of magnetization of the free magnet is changed to a new direction of magnetization, by a spin current generated by transfer of heat between at least the spacer and the pinned magnet. Thereafter, the new direction of magnetization of the free magnet is sensed. Many such structures are fabricated to have an easy axis of magnetic anisotropy in the free magnet, to implement memories that write data by transferring heat. Several such structures are fabricated to have an easy plane of magnetic anisotropy in the free magnet, to implement oscillators that generate an oscillating signal, on transfer of heat.

CROSS-REFERENCE TO PRIORITY APPLICATIONS

This application claims the benefit of and priority to U.S. ProvisionalPatent Application No. 61/285,332, filed on Dec. 10, 2009, by JohnCasimir Slonczewski as the inventor, entitled “Magnetic Oscillator andRandom-Access Memory Cell” and which is incorporated herein by referencein its entirety.

This application also claims the benefit of and priority to U.S.Provisional Patent Application No. 61/368,352, filed on Jul. 28, 2010,by John Casimir Slonczewski as the inventor, entitled “Method ofCreating Spin-Transfer Torque in a Magnetic Oscillator and aRandom-Access Memory Cell” and which is incorporated herein by referencein its entirety.

This application further claims the benefit of and priority to U.S.Provisional Patent Application No. 61/368,540, filed on Jul. 28, 2010,by John Casimir Slonczewski as the inventor, entitled “MaterialCompositions for Thermagnonic Spin Transfer Torque” and which isincorporated herein by reference in its entirety.

BACKGROUND

Spintronics relates to use of spin of electrons in various types ofdevices, such as oscillators and random access memory (RAM). Severalsuch prior art devices use an electrically resistive layer between twolayers of metallic magnets, to form a magnetic tunnel junction (MTJ).One of the just-described two metallic magnets has its magnetizationfixable into one of two directions in a plane of the layer of themagnet, depending on a direction of electric current. The magnetizationof the other of the two metallic magnets can rotate in the plane of thelayer. Passage of an electric current through the fixable-magnetizationlayer generates a spin current, whose absorption in therotatable-magnetization layer gives rise to a spin transfer torque thatchanges the direction of magnetization of the rotatable-magnetizationlayer.

For additional details on such a three layered structure, see an articleentitled “Magnetic-field Tunnel-sensor” by J. C. Slonczewski publishedin the IBM Technical Disclosure Bulletin. vol. 19. No. 6. Nov. 1976. pp.2331-2332, which is incorporated by reference herein in its entirety asbackground.

In several devices of the prior art, electric current flows through anelectrically conducting polarizing magnet, to generate a spin currentwhose absorption gives rise to a torque in a free magnet. As additionalbackground, the following patents are incorporated by reference hereinin their entirety:

(a) U.S. Pat. No. 5,695,864 granted to Slonczewski on Dec. 9, 1997, andentitled “Electronic device using magnetic components”;

(b) U.S. Pat. No. 7,149,106 granted to Mancoff, et al. on Dec. 12, 2006,and entitled “Spin-transfer based MRAM using angular-dependentselectivity”;

(c) U.S. Pat. No. 7,808,330 granted to Fukuzawa, et al. on Oct. 5, 2010and entitled “High-frequency oscillator”;

(d) U.S. Pat. No. 6,771,534 granted to Stipe on Aug. 3, 2004, andentitled “Thermally-assisted magnetic writing using an oxide layer andcurrent-induced heating.”

As further background, the following articles are also incorporated byreference herein in their entirety:

(f) Hatami et al, Phys. Rev. Lett. 99, 066603 (2007) “ThermalSpin-Transfer Torque in Magnetoelectronic Devices”; and

(g) Yu et al, Phys. Rev. Lett. 104, 146601 (2010) “Evidence of thermalspin-transfer torque.”

SUMMARY OF THE DISCLOSURE

A structure in accordance with the invention includes an electricallyconductive material possessing spontaneous magnetization (also called“free magnet”) not in contact with an electrically resistive materialpossessing spontaneous magnetization (also called “pinned magnet”), anda spacer having free electrons to transfer spin between the electricallyresistive material and the electrically conductive material. Duringoperation, a first direction of magnetization of the free magnet ischanged to a second direction of magnetization, by a spin current thatis generated by transfer of heat between at least the spacer and thepinned magnet. Thereafter, the second direction of magnetization of thefree magnet is sensed in one or more structures in accordance with theinvention. Many such structures are fabricated to have an easy axis ofmagnetic anisotropy in the free magnet, to implement memories that writedata, by transfer of heat in accordance with the invention. Several suchstructures are fabricated to have an easy plane of magnetic anisotropyin the free magnet, to implement oscillators that generate anoscillating output signal, upon transfer of heat in accordance with theinvention.

It is understood that other aspects will become readily apparent tothose skilled in the art from the following detailed description,wherein it is shown and described various aspects, embodiments, devicesand methods by way of illustration. The drawings and detaileddescription are to be regarded as illustrative in nature and not asrestrictive.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1A illustrates a structure in the form of a stack 110 in accordancewith the invention that includes a free magnet 114 not in contact with apinned magnet 112, and a spacer 113 located in direct contact with eachof magnets 112 and 114.

FIGS. 1B and 1C respectively illustrate flow of heat and spin throughstack 110 of FIG. 1A.

FIG. 2A illustrates another stack 120 in accordance with the inventionthat also includes magnets 112 and 114 and spacer 113 arranged in adifferent order relative to the order shown in FIG. 1A.

FIGS. 2B and 2C respectively illustrate flow of heat and spin throughstack 120 of FIG. 2A.

FIGS. 3A and 3B illustrate, in a plan view and a cross-sectional frontview respectively, an oscillator in some aspects of the invention, inthe respective directions 3A-3A (in FIG. 3B) and 3B-3B (in FIG. 3A).

FIG. 3C illustrates in a cross-sectional perspective view, theoscillator of FIGS. 3A and 3B.

FIGS. 4A and 4B illustrate, in a plan view and a cross-sectional frontview respectively, a portion of a memory in some aspects of theinvention, in the respective directions 4A-4A (in FIG. 4B) and 4B-4B (inFIG. 4A).

FIG. 4C illustrates in a cross-sectional perspective view, the memoryportion of FIGS. 4A and 4B.

FIG. 5A illustrates directions of spin polarization in some illustrativestructures in accordance with the invention.

FIG. 5B illustrates a relationship between the directions of spinpolarization of FIG. 5A.

FIG. 6A illustrates, in a flow chart, acts performed during operation ofseveral structures in accordance with the invention.

FIGS. 6B-6K illustrate, in cross-sectional views, change in the spinmoment vectors 610 and 613 when performing the acts illustrated in FIG.6A.

FIGS. 7A and 7B illustrate, in cross-sectional views, application ofheat to change the direction of spin moment vector 713, in accordancewith the invention.

FIG. 7C illustrates, in a cross-sectional view, location of a heatingelement 718 at the bottom of a stack in accordance with the invention.

FIGS. 8A-8C illustrate, in cross-sectional views, alternative locationsfor a heating element in accordance with the invention.

FIGS. 9A-9E illustrate, in cross-sectional views, various structures inseveral aspects in accordance with the invention.

FIGS. 10A and 10B illustrate, in a plan view and a cross-sectional frontview respectively, an oscillator in some aspects of the invention, inthe respective directions 10A-10A (in FIG. 10B) and 10B-10B (in FIG.10A).

FIGS. 11A and 11B illustrate, in a plan view and a cross-sectional frontview respectively, a memory cell in some aspects of the invention, inthe respective directions 11A-11A (in FIG. 11B) and 11B-11B (in FIG.11A).

DETAILED DESCRIPTION

The detailed description set forth below in connection with the appendeddrawings is intended as a description of various aspects of the presentdisclosure and is not intended to represent the only aspects in whichthe present disclosure may be practiced. Each aspect described in thisdisclosure is provided merely as an example or illustration of thepresent disclosure, and should not necessarily be construed as preferredor advantageous over other aspects. The detailed description includesspecific details for the purpose of providing a thorough understandingof the present disclosure. However, it will be apparent to those skilledin the art that the present disclosure may be practiced without thesespecific details. In some instances, well-known structures and devicesare shown in block diagram form in order to avoid obscuring the conceptsof the present disclosure. Acronyms and other descriptive terminologymay be used merely for convenience and clarity and are not intended tolimit the scope of the disclosure.

Some aspects of this invention provide a highly efficient method ofcreating spin-transfer torque for use in magnetic oscillators andrandom-access memories. In many aspects of the invention, an externallysourced flow of heat through an electrically resistive first magnet(also called “polarizing” magnet or “pinned” magnet or “magnonicpolarizer”) creates a current of electron-spin polarization that flowsthrough a conducting normal non-magnetic metal spacer into a second,metallic magnet (also called “free” magnet). This spin current creates atorque that excites the magnetic moment of this second magnet into astate of precession. Making this precession steady by providing a uniqueeasy plane of magnetic anisotropy creates a spintronic magneticoscillator in accordance with this invention. Making this precessiontransient by providing a unique easy axis of magnetic anisotropy leadsto switching of the second moment for the write operation in a magneticmemory cell in accordance with this invention. Both of the two genericversions (named Version #1 and Version #2) in accordance with thepresent invention pertain to those spintronic oscillators andrandom-access memories (STT-MRAMs) that rely on a spin-transferphenomenon to exert torque on the spontaneous spin moment of a metallicmonodomain free magnet.

As readily apparent to the skilled artisan, STT-MRAM is an abbreviationfor “spin transfer torque magnetic random access memory.” As noted abovein the background section, in prior art, electric current that flowsthrough an electrically conducting polarizing magnet generates the spincurrent whose absorption gives rise to a torque in the free magnet.

Instead of electricity, several devices in accordance with the presentinvention rely on the flow of heat through an electrically resistivepolarizing magnet (also referred to herein as a magnonic polarizer), togenerate the spin current that creates the needed torque. In anillustrative aspect of the present invention, dubbed magnonic (orequivalently thermagnonic) spin-transfer (MST), an externally providedelectric current may create the necessary heat by means of Joule heating(also called Joule effect, ohmic heating or resistance heating).However, it is the flow of this heat energy carried by magnons inseveral aspects of the invention, rather than the electric currentcarried by electron movements known in prior art, which physicallyexcites the flow of spin-polarization from within the magnonicpolarizer. This change in operation provides an increase, over priorart, of the spin-transfer torque per unit electric current supplied inthe device, making it possible for an oscillator or a spintronic memorycell in accordance with this invention to function with some combinationof smaller drive current, greater oscillator frequency, and/or greaterwriting speed. In addition, several aspects of this invention increasethe opportunity for favorable scaling of devices relying on spintransfer. Scaling an oscillator in accordance with this invention, togreater size increases its power output; scaling an STT-MRAM cell inaccordance with this invention, to smaller size increases the memorydensity.

In some aspects of the invention, the invented magnonic spin-transfer(MST) method of creating magnetization torque uses a stack of fivematerial elements or layers contributing specific functions. In bothVersions #1 (see stack 110 in FIG. 1A) and #2 (see stack 120 in FIG. 2A)of MST, an element 111 called a heater is located at one extremity (e.g.top) of this stack. An externally powered source creates heat that flowsfrom this heater 111 in sequence through the remaining layers of thestack. Our terminology “heater” may include a thermal barrier to preventflow of heat away (upward in FIGS. 1A-1C, 2A-2C, 3B and 4B) from theremainder of the device, if the use of the invention needs it.

In some structures in devices in accordance with the invention, astructure in the form of a stack is formed of layers although as will bereadily apparent in view of this disclosure other structures also inaccordance with the invention use elements which do not have the formfactor of layers but are formed of the same materials as the layers.Hence, depending on the aspect of the invention, two or more suchelements may be positioned relative to one another so as to interoperatein a manner similar or identical to operation of corresponding layersdescribed herein. Accordingly, although some description of variousdevices below refers to one or more layers, it is to be understood thatcorresponding one or more elements are similarly used in several suchdevices that are also contemplated by the inventor as being inaccordance with the invention.

In Version #1 (see FIG. 1A) of magnonic spin transfer, said heat flowsfrom the heater 111 down into the second layer 112, the saidelectrically resistive ferro- or ferri-magnet serving as magnonicpolarizer. In such a magnet 112, each quantized spin excitation, calleda magnon, carries a quantum of heat energy together with a quantum ofelectron spin that is polarized oppositely to the spontaneous spinmoment of the magnet. These heat and spin currents continue through thethird layer 113, a normal non-magnetic metal spacer, in which atomicallyunbound electrons freely carry both the heat and spin currents. Bothheat and spin currents continue into the fourth layer 114, the saidmetallic free magnet. Absorption of this spin current by the free magnet114 creates the desired spin-transfer torque acting on its spin moment.

The torque, thus created by MST, excites the magnetic moment of the freemagnet 114 into the precessional oscillation that provides theoscillator or memory-write functions similar or identical tocorresponding functions described in the prior art for current-drivenspin transfer. The fifth and last layer 115 of said stack 110 isthermally conductive and sufficiently bulky to disperse the heat flowinginto it without substantially impeding the heat flow or adverselyaffecting other circuit elements that may be present nearby. In oneillustrative example in accordance with the invention, a siliconsubstrate normally used in manufacturing of structures in integratedcircuits is included in the fifth element 115 of stack 110. In anotherillustrative example in accordance with the invention, substrate ofmetal is included in the fifth element 115 of stack 110.

Hence, a stack 110 in several devices of the invention includes a heatsink having greater thermal conductance than the electrically resistivematerial in the second layer 112. Depending on the embodiment, the heatsink 115 is located in direct thermal contact with at least one ofelectrically resistive material in layer 112 (see FIG. 2A), theelectrically conductive material in layer 114 (see FIG. 1A), and thespacer formed by layer 113 (not shown).

Note that electric current plays no inherent role in the abovedescription of magnonic spin transfer. In several aspects of theinvention, any spin current that might be associated more directly withsome electric current existing in a particular use of MST contributesless in order of magnitude to the torque on the moment of the freemagnet 114; the present description of MST neglects it (the torque).

Version #2 (see stack 120 in FIG. 2A) of the magnonic spin-transfer insome aspects of the invention uses material layers just like those ofVersion #1 described above (see stack 110 in FIG. 1A). However, theselayers are arranged in a different order. In Version #1, heat flowsfirst through the magnon polarizer 112, then through the free magnet 114(see FIG. 1B). However, the different order of the layers in Version #2provides heat flow first through the free magnet 114, then through themagnon polarizer 112, as depicted in FIG. 2B. The difference betweenVersions #1 and #2 in order of polarizer 112 and free magnet 114 causesa difference in the direction of torque on the moment of the free magnet114, for the same material compositions.

Although a free magnet has been described above as being metallic, anymaterial that is electrically conductive and possess spontaneousmagnetization (or residual magnetization) may be used as a free magnetin various devices in accordance with this invention. In numerous suchdevices, the above-described polarizing magnet (or magnonic polarizer,or magnon source) is formed of a material that possesses spontaneousmagnetization, but this magnetic material is electrically resistive inbulk (at least relative to the electrically conductive material used inthe free magnet). In some aspects of the invention, a magnetic materialthat is included in the polarizing magnet has an electrical conductancetherethrough that is negligible (e.g. three or four orders of magnitudelower) relative to the electrical conductance through the free magnet.

A magnetic material that is included in the polarizing magnet isselected (by design) to have an electrical conductance that issufficiently negligible to ensure that all (or almost all, or most, ormajority, or substantial amount, depending on the aspect of theinvention) of the heat energy transferring between the spacer and thepolarizing magnet is carried by magnons, rather than by electronmovement. Thus, as will be readily apparent in view of this disclosure,various polarizing magnets in accordance with this invention may havevarying amounts of electrical conductance, and various devices inaccordance with the invention that use such polarizing magnets havevarying degrees of effectiveness when using heat (instead of electriccurrent) to transfer magnetization direction from the polarizing magnetto the free magnet.

In several aspects of the invention, the polarizing magnet and the freemagnet do not touch each other and instead, a spin current istransferred therebetween via a spacer as described above. Although anormal metal spacer is described above, a spacer that is used in manydevices in accordance with this invention may include any material (suchas copper) that is diamagnetic, i.e. having no unpaired spins, e.g.wherein for each electron having an up spin, there exists anotherelectron that has a down spin. As will be readily apparent in view ofthis disclosure, spacers in accordance with this invention may include,in addition to a diamagnetic material, varying amounts of one or moreimpurities (e.g. embedded therein) and such impurities may haveelectrons with unpaired spin. Therefore, some devices in accordance withthe invention that use spacers with impurities have varying degrees ofeffectiveness in transferring therethrough the spin current (and thusthe magnetization direction).

In several aspects of the invention, each of the two illustrative, hereinvented device Uses (#1 and #2) of the said MST method (i.e. stacks 110and 120) requires the incorporation of a sensor which may be similar oridentical to a corresponding sensor described in the prior art (seeBackground section above), namely a tunnel barrier separating a freemetallic magnet from a second metallic magnet having a static momentvector. In addition, in some aspects of the invention, each Useincorporates a tunneling heater and thermal barrier available in theprior art for thermally assisted memory (TA-MRAM).

Depending on the aspect of the invention, such a heater is positioned ina structure in accordance with the invention so as to transfer heat atleast between the spacer and the electrically resistive material.Moreover, in some aspects of the invention, the thermal barrier has asmaller thermal conductance than at least the electrically resistivematerial 112 (see FIG. 8A).

Illustrative devices in certain aspects of the invention include atunneling heater of the type described in U.S. Pat. No. 6,385,082granted to Abraham et al, entitled “Thermally-assisted magnetic randomaccess memory (MRAM)” which is incorporated by reference herein in itsentirety. Several aspects of the invention include a sensor of the typedescribed in, for example, U.S. Pat. No. 7,411,817 granted to Noziereset al entitled “Magnetic memory with a magnetic tunnel junction writtenin a thermally assisted manner, and method for writing the same” whichis incorporated by reference herein in its entirety. Additionally, insome aspects of the invention, such illustrative devices include athermal barrier (e.g. containing Tantalum), of the type described byPapusoi et al, in New Journal of Physics 10 103006 (2008), an articleentitled “Probing fast heating in magnetic tunnel junction structureswith exchange bias” which is incorporated by reference herein in itsentirety.

In certain aspects of the invention, Use #1 in an oscillator (see FIGS.3A-3B) combines Version #2 of MST (see FIG. 2A) with elements taken fromprior art. See, for example, U.S. Pat. No. 7,411,817 granted to Nozieresincorporated by reference above. Various prior art for said TA-MRAMsupplies the entire pillar 310 in the form of a circular cylinder (seeFIG. 3C). Within this pillar 310 (see FIG. 3B), the antiferromagnet 311pins the moment of the “pinned metallic magnet” 312 in some fixedhorizontal direction making the prior-art sensor 313 sensitive to onehorizontal component of the moment of the free magnet 314. Thetunnel-bather component 315 of this pillar 310 serves as the heaterelement 111 (see FIG. 2A) required in Version #2 of MST of someembodiments of the invention. The thermal barrier 316, known in theprior art for TA-MRAM, prevents heat from flowing upward, thus insuringthat most of it flows downward into the remainder of the device 300 (seeFIG. 3B). The free magnet 314 of prior sensor art, located at the bottomof the cylindrical pillar 310, has uniaxial magnetic anisotropy energywith an easy horizontal plane and serves as the second layer 114 (seeFIG. 2A) of MST Version #2 in several aspects of the invention.

Note that all directions described herein, such as “bottom” in theprevious paragraph, as well as “below”, “vertically”, and “upward” inthe next paragraph, are being described relative to an orientation of adevice shown in a corresponding figure attached hereto that is beingdescribed herewith.

In several aspects of the invention, the portion of the oscillatorstructure 300 (see FIG. 3B) that lies below the cylindrical pillar 310includes the three remaining layers 317-319 required by Version #2 ofMST (e.g. stack 120 in FIG. 2A). The lateral dimensions (e.g. width andlength) of these remaining layers 317-319 may be greater than thediameter of the pillar (e.g. see Dp in FIG. 3B, which may be, forexample, 100 nm). Of these three remaining layers, the “base electrode”layer 317 serves as the “normal metal spacer” (third layer 113) of MSTVersion #2. The “pinned resistive magnet” (see layer 318 in FIG. 3B) hasits remanent spin moment directed vertically and serves as the “magnonicpolarizer” (fourth layer 112) of MST Version #2. This upward directionof spin moment provides an upward component of spin-transfer torque tothe spin moment of the free magnet 314 when heat flows from the tunnelbather 315 (acting as a heater) through the free magnet 314. Finally,the “silicon CMOS substrate” 319 provides the thermal-dispersion element(fifth layer 115) included in MST. In addition, this substrate 319includes the prior-art integrated circuitry (not shown) necessary foroperation of the oscillator.

In some aspects of the invention, operation of oscillator 300 shown inFIG. 3B requires the supply from the CMOS substrate (see item 319 inFIG. 3B) of a steady electric current through device 300, betweenterminals T1 and T2. Note that in the illustrative device shown in FIG.3B, a terminal T1 that is external to oscillator 300 is coupled toelectrode 321 (also called “pillar” electrode) of pillar 310 whileanother such externally-accessible terminal T2 is coupled to baseelectrode 317 (described above). This current serves two functions: Onefunction is to power the said prior-art tunneling heater 315, whichsupplies, by MST Version #2 (see FIG. 2B), the torque that sustainsprecession of the moment of the free magnet 314 (see FIG. 3B). Itssecond function is to bias the magnetic tunnel junction 300 so that, asknown from prior art for magnetic tunnel junction sensors, the desiredoscillating component of output voltage created between the said pinnedand free metallic magnets 318 and 314 appears between external terminalsT1 and T2 (see FIG. 3A).

Use #2 of MST provides the write operation in a rectangular MRAM array400 (see FIG. 4A) arranged in rows and columns of memory cells. Any onesuch cell (see cell 410 in FIGS. 4A and 4B) contains Version #2 of hereinvented MST together with prior-art features including a heater 411with thermal barrier 412, magnetic-tunnel junction sensor 413, andreversal of polarizer moment by means of a current-induced magneticfield.

The pillar 414 indicated in FIG. 4B has an elliptic cross-section. Itcontains said thermal barrier 412 insuring that most of the Joule heatcreated by electric current flowing through the tunnel-barrier heater411, which forms a part of the magnetic-tunneling sensor 413, flowsdownward (relative to FIG. 4B) through the remainder of the memory cell410. Thus said tunnel barrier 411 (see FIG. 4B), together with the saidthermal barrier 412, serve as the “heater” that is the first element 111of MST Version #2 (see FIG. 2A). The free magnet 415 at the bottom ofsaid pillar 414 is its second element 114. It (free magnet 415) storesone bit of data, 0 or 1, represented by the two stable magnetizationdirections along a major axis of the elliptic cross-section (of pillar414).

A “short metal strip” 416 (see FIG. 4B) that serves as the “normal metalspacer” (third layer 113) of MST Version #2 (see FIG. 2A) supports thesaid elliptic pillar 414 of each memory cell 410 such as the one shownin FIG. 4B. Beneath the many short metal strips (see 416 in FIGS. 4A,4B) present in the memory array 400 lies the nearly continuous“resistive magnet” 417 that serves as the magnonic polarizer (fourthlayer 112) of Version #2. The vertical “cylindrical electrode” 418threads the via hole in said resistive magnet 417 to provide electricconnection between the said short metal strip 416 of each cell and theCMOS substrate (not shown); aside from such via holes, the polarizer 417is continuous underneath the entire memory array. These via holesinclude one hole for each memory cell in order to connect terminal T2 toits assigned selecting transistor or diode present within the CMOS orother semiconductor chip that supports the entire memory array 400. The“long metal strip” 419 that lies under a column of data cells (shown bydots along a vertical line 420 in FIG. 4A) extends in a directionorthogonal to the plane of the front view in FIG. 4B. The said longmetal strip 419 serves as the thermal disperser (fifth layer) of MST(see FIG. 2A) in some aspects of the invention.

To write a single bit of data into the memory cell illustrated in FIGS.4A-4C, one provides a pulse of electric current I1 through the said longmetal strip 419. This current induces a magnetic field that orients themoment vector of that portion of the resistive magnet 417 that liesdirectly underneath the column of data cells 420 that includes this cell410. The resulting moment direction, either one of two directions thatdepends on the sign of current I1, determines whether a 0 or 1 data bitis to be written. To complete the write operation, a prior-arttransistor or diode that is located in underlying CMOS (not shown) andis connected to terminal T2, selects the cell 410 by causing a currentpulse I2 that overlaps I1 in time, to flow between terminals T1 and T2,thus powering the heater 411. The resulting flow of heat switches thefree magnet 415 in accordance with Version #2 of magnonic spin transfer(see FIG. 2B). Subsequently, the bit of stored data may be read by meansof a weaker current flowing between terminals T1 and T2 of themagnetic-tunnel-junction sensor.

Next are described essential elements and workings of the two Versions,#1 and #2 (see FIGS. 1A-1C and 2A-2C), in some aspects of magnonicspin-transfer (MST) in accordance with the invention. Then are describedalternative uses of the MST in several aspects of the invention, namelya spintronic oscillator device (see FIG. 3B) and a cell of random-accessmagnetic memory (see FIG. 4B). Each of these two versions of MST (seeFIGS. 1A-1C and 2A-2C) has five functioning layers 111-115 that includean unspecified externally powered heater. To create heat, one may useJoule heating due to an electric current, or a focused electron beam, ora focused light beam, or a nanoscopic light-emitting diode (FIG. 9C), orsliding friction, or a chemical reaction such as combustion.

FIG. 1A illustrates version #1 of MST stack 110 in some aspects of theinvention, specifying the direction of heat flux F from the magnonicpolarizer 112 to the free magnet 113 that receives the transferred spinmomentum. As noted above, the sequence of five layers in FIG. 1Aconsists of heater 111, magnonic polarizer 112, normal metal spacer 113,free magnet 113, and thermal disperser 114. In many aspects of theinvention, neighboring elements (e.g. polarizer 112 and spacer 113) haveionic bonds to one another, in order to ensure easy heat flowtherebetween. The thicknesses of these layers shown in FIG. 1A are notdrawn to scale. In several such aspects of the invention otherneighboring elements (e.g. free magnet 114 and spacer 113) have metallicbonds to one another. If a semiconductor material is used in an elementof a device in accordance with the invention, such an element may havecovalent bonds with another element in the device.

Here follows a description of these five layers in several aspects ofthe invention. In FIG. 1A, the said heater 111 connects directly to amagnonic polarizer 112. In certain aspects of the invention, the“magnonic polarizer” used in FIG. 1A means a ferromagnet or ferrimagnethaving a spontaneous magnetization and sufficient electric resistancethat the flux of heat carried by movement of conduction electronsconstitutes none or only a minor part of the entire heat flux. In someaspects of the invention, magnetic quantum excitations known in physicsas magnons carry a major portion of the heat through said magnonicpolarizer 112. Therefore, the metallic elements or alloys commonly usedas (electric) spin polarizers in the prior art for spin transfer are notsuitable (in some aspects of the invention) because electron motions,rather than magnons, transport most of the heat through them.

Preferred alternative compositions for an in-plane magnetized magnonicpolarizer 112 include the insulating polycrystalline ferrites having thecompositions meghamite γFe₂O₃, manganese-magnesium ferrite (Mn,Mg)Fe₂O₄,lithium-iron ferrite Li_(O.5)Fe_(2.5)O₄, lithium-zinc ferrite(Li,Zn,Fe)₃O₄, yttrium iron garnet Y₃Fe₅O₁₂, yttrium-gadolimium irongarnet (Y,Gd)₃Fe₅O₁₂, and nickel ferrite NiFe₂O₄. In several aspects ofthe invention, the just-described compositions in this paragraph areincluded in fixed magnets that are used to implement memory cells of thetype described herein.

Preferred alternative compositions for a plane perpendicular magnetizedmagnonic polarizer 112 include the c-axis textured insulatingcompositions barium hexaferrite BaFe₁₂O₁₉, barium gadolinium-ironhexaferrite Ba(Gd,Fe)₁₂O₁₉, and aluminum-iron hexaferriteBa(Fe,Al)₁₂O₁₉. In several aspects of the invention, the just-describedcompositions in this paragraph are included in fixed magnets that areused to implement oscillators of the type described herein.

Every one of the magnetic ions (Mn²⁺, Fe³⁺, Gd³⁺) present in each of thesaid preferred alternative compositions of the magnonic polarizer 112 insome aspects of the invention has the orbital quantum number L=0.Consequently, each preferred composition has weak spin-phononrelaxation. Therefore, these ferrites communicate little heat betweenthese ionic spins and the phonons whose thermal conductance wouldotherwise weaken the desired spin transfer.

In FIG. 1A, the term “normal metal spacer” denoting the third layer 113means (in several aspects of the invention) a good electrical conductor,such as copper or silver, which contains no atomic spin moments to causespin relaxation. The term “free magnet” specifying the fourth layer 114is well known in the prior art for spin-transfer magnetic oscillatorsand random access memories (STT-MRAMs) as an element or alloy thatincludes at least one of the elements Fe, Co, Ni, and, in additionperhaps B (boron). The term “thermal disperser” denoting the fifth andlowest layer 115 means a material like silicon or any metal having ahigh thermal conductivity. In addition, the thermal disperser 115 isthicker and has at least one large (e.g. an order of magnitude large)horizontal dimension relative to those of the free magnet 114 in orderto disperse efficiently the heat created by the heater 111 withoutsignificantly affecting other devices that might be in contact with it.

In several aspects of the invention, to perform a magnonic spin-transferfunction, one applies either a single pulse of heat or a steady heatingby means of the said heater 111. As a result, heat flux in amount F,here considered steady, flows through the entire device 110 (or 120)from the heater 111 into the said thermal disperser 115, as depicted inFIGS. 1B and 2B.

Referring to FIG. 1C, consider a Cartesian axis z along the direction ofthe spontaneous electronspin moment vector Smp of the said magnonicpolarizer 112. Although Smp and axis z appear oriented vertically upwardin FIG. 1C, depending on the aspect of the invention, they may actuallytake any common direction in three dimensions with respect to thedepicted five-layer device geometry of device 110 (or device 120).Passage of this heat flux through the interface 191 (see FIG. 1A)between the magnonic polarizer 112 and the said normal metal spacer 113annihilates incident magnons and transfers their z-component ofquantized electron spin, having the negative Planck amount h/2π321.05×10⁻³⁴ Js, to the electrons in the normal metal spacer. Thistransfer of spin component to the normal metal spacer 113 creates acondition known in physics as spin accumulation that, in the xz-space ofFIG. 1C, has negative algebraic sign along the z-axis.

As understood from the prior art for certain devices using spin transferdriven by electric current, this spin accumulation driven by heat in thepresent invention stimulates the positive flow Q_(z) (>0) of z-componentspin momentum indicated schematically by the vertical arrows inside thedevice shown in FIG. 1C. Boldface characters “X” shown within the heater111 and magnonic polarizer 112 regions indicate that thermal relaxationof spin polarization within one or both of them replenish the magnonsand their spins emitted, as described above, into the polarizer-metalinterface.

While heat is flowing, the said free magnet 114's spin moment S_(fm) maytake a range of instantaneous directions in three dimensions. Forconvenience of description, FIG. 1C shows only the special instant thatS_(fm) lies along the Cartesian x-axis orthogonal to Smp. In thisspecial case, the free-magnet moment fully absorbs the negative of thecomponent Q_(z) of spin momentum current impinging on it, as indicatedby the boldface character “O” seen in FIG. 1C. The result is that thez-component of S_(fm) acquires a time derivative dS_(fm,z)/dt=Q_(z),interpretable as a downward effective spin-transfer torque τ=dS_(fm)/dton free-magnet spin vector S_(fm) indicated in FIG. 1C.

For a general angle θ (see FIG. 5B) between S_(fm) and S_(mp), themagnitude of said torque is given by the relation |dS_(fm)/dt|=|Q_(z)sin θ|. Just as in the prior art for spin transfer driven by electriccurrent, in many aspects of this invention said torque excites thefree-magnet moment to effect its precession about an axis of magneticanisotropy. The use of MST in an oscillator in some aspects of theinvention described below requires that this precession is steady. Inits use in a memory cell also described below for several aspects of theinvention, this precession of S_(fm) is transient, leading ultimately toswitching of the moment between two directions representing twoinformation states.

FIG. 1C indicates that the direction of torque dS_(fm,z)/dt=−Q_(z) tendsto orient the said moment S_(fm) into the direction opposite to saidpolarizer moment S_(mp). We call this Version #1 case of MST divergent.In the present Version #1 with heat flow direction from magnonicpolarizer 112 to free magnet 114, the torque must be divergent inseveral aspects of the invention.

FIG. 2A illustrates a structure that uses Version #2 of MST in someaspects of this invention. FIG. 2B depicts the direction of heat flux Ffrom the free magnet 114 to the magnonic polarizer 112 in the structureof FIG. 2A. The above discussion of Version #1 (see FIGS. 1A-1C) alreadydescribed its same five component layers 111-115 (see FIGS. 2A-2C). Inthe aspects of the structure illustrated in FIGS. 2A-2C, Version #2 hasthe new sequence: heater 111, free magnet 114, normal metal spacer 113,magnonic polarizer 112, and thermal disperser 115.

FIGS. 2B and 2C, like FIGS. 1B and 1C, depict heat flow andspin-momentum flow, respectively. In this case the spin-transfer torquetends to align the said moment S_(fm) into the same direction as thesaid polarizer moment S_(mp). Hence this Version #2 of thermagnonic spintransfer is termed convergent.

In several aspects of the invention, the operation of magnonic spintransfer in Version #2 differs from that of Version #1 in the followingways: referring to FIG. 2C, flow of heat across the interface 191between the magnonic polarizer 112 and normal metal spacer 113 createsspin accumulation in the normal metal spacer 113. In some aspects of theinvention, the resulting spin-chemical potential difference across thisinterface 191 draws spin polarization out of the magnonic polarizer 112and thermal disperser 115 by inverse relaxation indicated in FIG. 2C bythe character O, and drives the spin-current component Q_(z) flowingfrom the disperser 115 to the free magnet 114. The moment of the freemagnet 114 absorbs this spin current to create apparent torque in suchaspects of the invention. As indicated, in certain aspects of theinvention, the torque direction is convergent; it tends to align thefree magnet moment S_(fm) with that of a ferromagnetic magnonicpolarizer S_(mp). Depending on the aspect of the invention, such atorque may drive magnetic dynamics in an oscillator or a memory celljust as in the case of Version #1.

In the prior art for spin-momentum transfer driven directly by electriccurrent, the occurrence of dynamic convergence versus divergence of thetwo magnetic moments varies with the sign of the driving current.

In the case of magnonic spin-momentum transfer in many aspects of theinvention, interchanging relative locations of the polarizer and freemagnet (Versions #1

#2) results in convergence replacing divergence for a given set ofmaterial compositions.

Depending on the aspect of the invention, those skilled in the art maycombine some of the five element functions of MST with functionspertaining to other device features, taking care to preserve thesequence of (two or more elements illustrated in) either Version #1 orVersion #2. Additionally, they may insert additional elements havingdifferent functions among those of MST, as dictated by the intendedusage. Such insertions are subject to two provisos either alone or incombination depending on the aspect of the invention. The first provisois that the additional elements do not substantially impede the flow ofheat between the heater 111 and the thermal disperser 115. The secondproviso is that the normal metal spacer 113 must remain in chemicallybound contact with both the magnonic polarizer 112 and the free magnet114 in order to insure unimpeded flow of spin current between them. Theexemplary uses of MST described below illustrate such liberties ofdevice design in various aspects of the invention.

Refer to FIGS. 3A-3C. In several aspects of the invention, an inventedoscillator device 300 has three connected geometric parts (1)-(3) asfollows. These parts are 1) a cylindrical pillar 310 of circularcross-section visible in the plan view (FIG. 3A). Its diameter Dp isabout 100 nm. The front view (FIG. 3B), in which the element thicknessesare not to scale, shows how this pillar 310 rests vertically on 2) abase 317 in the form of a rectangular parallepiped one or both of whoselateral dimensions (width and length shown in FIG. 3A) exceed thediameter Dp of the cylinder 310. This base 317 rests on 3) an evenlarger heat disperser 319 that may include a number of variousintegrated devices using Si technology such as CMOS.

Said pillar 310 (FIG. 3B) in total is adapted from the prior art for amemory cell that uses elevation of temperature to temporarily lower theenergy barrier due to magnetic anisotropy or exchange pinning of a freemagnet in order to allow its moment to switch in a modest appliedmagnetic field (as described in U.S. Pat. No. 7,411,817).

In several aspects of the invention, said pillar 310 is composed of sixelements visible in the front view shown in FIG. 3B. In downwardsequence, these elements are electrode 321 connected to terminal T1,thermal barrier 316, antiferromagnet 311, pinned metallic magnet 312,tunnel barrier 315, and free magnet 314. The antiferromagnet 311 pinsthe magnetic moment of the pinned magnet 312 along a horizontaldirection so that the magnetic tunnel junction (formed by pinned magnet312, free magnet 314 and tunnel barrier 315) senses an alternatingvoltage proportional to this same horizontal component of the precessingmoment of the free magnet 314.

Said pillar 310 already includes two of the five layered elements thatcomprise a Version #2 device (see FIG. 2A) of MST in some aspects of theinvention. Firstly, the tunnel barrier 315 together with thermal barrier316 comprise the heater 111 of MST; when electric current flows throughthe pillar 310 (FIG. 3B) from terminal T2 to terminal T1, electronstunnel quantum-mechanically at high voltage (circa 1 Volt) through thebarrier 315, thus depositing heat energy into the free magnet 314. Thesaid thermal barrier 316 resists upward heat flow, thus insuring thatmost of this generated heat flows downward into the free magnet 314 asrequired for MST (see FIGS. 2A and 2B). Secondly, this free magnet 314(FIG. 3B) at the bottom of the pillar 310 constitutes the second element114 of MST.

The front view in FIG. 3B also shows the three remaining elements ofmagnonic spin transfer that lie below the pillar 310, in certain aspectsof the invention. Uppermost is the base electrode 317 (FIG. 3B) thatserves as the normal metal spacer 113 (FIG. 2A) of MST. In some aspectsof the invention, at least one lateral dimension of the base electrode317 exceeds the diameter of the pillar 310 in order to provide externalelectric access to the pillar 310 through terminal T2. Additionally, inseveral aspects of the invention, this base electrode 317 is thin enough(10 nm or less) to limit (or reduce) heat loss in a lateral direction.

Under the base electrode 317 lies the pinned resistive magnet 318 (e.g.of thickness circa 10 nm) serving as the magnonic polarizer 112 of MST.It may have the same lateral dimensions as the base electrode 317. Inorder to provide a large torque on the free magnet 314 this polarizer318 is best a non-metallic ferrimagnet in many aspects of thisinvention. For example, deposition of c-axis vertically texturedcrystalline barium hexaferrite BaFe₁₂O₁₉, followed by application ofsufficient vertical magnetic field, will pin the moment vertically in amagnetically remanent state, in a manner similar or identical to priorart.

Finally, the device 310 includes the extended heat disperser 319 thatconsists of Si-based CMOS electronics similar or identical to prior art,and that may support a large number of integrated electronic and/orspintronic devices, e.g. in an integrated circuit.

Both views in FIGS. 3A and 3B show terminals T1 and T2 providingexternal electric access to the said pillar 310. Electronic controlcircuitry within the silicon substrate 319 supplies, through leads whichare not shown, constant electric current flowing from terminal T2 toterminal T1. In numerous aspects of the invention, a majority of theresulting heat flow generated by the tunnel bather 315 flows downwardthrough all five of the elements 111, 114, 113, 112 and 115 of Version#2 of MST. As described above, this heat flow creates a torque τ on thespin moment of the free magnet 314. This torque τ excites precession ofthe free-magnet moment vector about the vertical axis, through pillar310 in FIG. 3B. The resulting alternating term of the voltage is sensedbetween T1 and T2, similar or identical to prior-art for magnetictunneling sensors, thereby fulfills the purpose of an oscillator 300.

Refer to both plan and front views in FIGS. 4A and 4B. In severalaspects of the invention, an invented memory cell has six geometricallydistinct parts, as follows: The uppermost of these parts is acylindrical pillar 414, of elliptic cross-section visible in the topview in FIG. 4A. This pillar 414 has eight elements of prior-artmaterials (individually described below) that are visible in the frontview in FIG. 4B. This front view, in which the element thicknesses arenot to scale, shows how this pillar 414 rests vertically on therectangular short metal strip 416 that extends laterally. Its width Swmay exceed the corresponding minor diameter Ed of the said ellipticcross-section.

The rectangular array (not shown) of said pillars and short metal stripsrests on a single element 417 of electrically resistive magneticmaterial that extends great distances in both lateral directions so thatit supports one or more rectangular arrays of memory cells (not shown).A element 419 of long parallel metal strips, like the one shown in bothviews of FIGS. 4A and 4B, extends orthogonally through the plane of thepaper in FIG. 4B and lies under a column of such cells. One verticalcylindrical electrode 418 threads through a via hole 417V in theresistive magnet 417 in order to connect, via terminal T2, said shortmetal strip 416 with a transistor or diode present in a element of 6)prior-art CMOS circuitry (not shown) that supports all of the memorycells.

In several aspects of the invention, said pillar 414 is adapted from theprior art pillar for a memory cell that uses elevated temperature tolower the energy barrier of a free magnet to switching by a magneticfield. However, the here invented memory cell makes no use oftemperature elevation; instead, by means of MST explained above, it usesthe flow of heat to create spin-transfer torque τ.

In some aspects of the invention, the said pillar 414 is composed ofeight elements visible in the front view of FIG. 4B. In downwardsequence, these elements consist of the following: electrode 421surmounted by terminal T1, thermal barrier 412, antiferromagnet 422,pinned magnet #1 423, normal metal 424, pinned magnet #2 425, tunnelbarrier 411, and free magnet 415. The antiferromagnet 422 holds themagnetic moment of pinned magnet #1 423 that in turn pins, by means ofnegative exchange coupling through the normal metal spacer 424, pinnedmagnet #2 425 along the major axis of the elliptic cross-section(horizontal in FIGS. 4A and 4B). Consequently, in a manner similar oridentical to prior art, any flow of electric current between terminalsT1 and T2 causes a voltage that depends on which of the two equilibriumdirections, along the major elliptic axis, that the magnetic moment ofthe free magnet 415 occupies. These two equilibrium directions of thefree-magnet moment represent the digital 0 and 1 states of the memoryelement 410.

In certain aspects of the invention, said pillar 414 visible in thefront view of FIG. 4B includes two of the five elements that compriseMST Version #2 described above and illustrated in FIG. 2B. Firstly, thesaid tunnel barrier 411 and said thermal barrier 412 in FIG. 4B togethercomprise the heater 111 of MST; when electric current flows through thepillar 414 when flowing from terminal T2 to terminal T1, electronstunnel quantum-mechanically at high voltage (circa 1V) through thebarrier 411, thus depositing heat energy in the free magnet 415. Thesaid thermal barrier 412 prevents upward heat flow, thus insuring thatthis generated heat flows downward into the remaining elements of MST.Additionally, the free magnet 415 at the bottom of the pillar 415 alsoconstitutes the second element 114 of MST (see FIG. 2A).

In several aspects of the invention, the front view in FIG. 4B shows thethree remaining elements of MST lying below the pillar 414. Uppermost isthe short metal strip 416 that serves as the normal metal spacer (thirdelement) 113 of MST (see FIG. 2A). Terminal T2 that connects to thisstrip 416 (see FIG. 4B) via the vertical cylindrical electrode 418provides, together with terminal T1, the external electric access to theillustrated memory cell 410. Additionally, this electrode strip 416 isthin enough (10 nm or less) to limit heat loss in a lateral direction.

In some aspects of the invention, underneath said short metal strip 416lies the nearly continuous resistive magnet 417 serving as the magnonicpolarizer (fourth element) 112 required for MST (see FIG. 2A). One causeof departure from its continuity is the requirement of one via hole 417Vfor each cell 410 in order to allow electric connection (not shown) ofterminal T1 to the control circuitry in the CMOS substrate at the bottomof the memory plane. To provide a large torque τ on the free magnet 415,in certain aspects of the invention this polarizer 417 is made of anon-metallic composition in order to avoid heat transport via conductionelectrons. Preferably, deposition of any of the in-plane magneticallypermeable crystalline ferrites said above, in the description of MST,serves this purpose.

Finally, in several aspects of the invention said resistive magnet 417rests on the set of parallel long metal strips (such as strip 419 inFIG. 4B) that constitute the extended heat disperser (fifth element) 115of MST (see FIG. 2A) for the whole memory array. Strip 419 in someaspects of the invention is multiple times longer than any dimension ofthe electrically resistive material in magnet 417, strip 419 beinglocated sufficiently close to magnet 417 such that a direction ofmagnetization of the magnet 417 is changed on passage of an electricalcurrent through strip 419.

The magnetic shape anisotropy due to the elliptic cross-section of saidpillar 414 (FIG. 4A) provides the two easy 0 and 1 equilibriumdirections of the free-magnet moment vector along the major ellipticaxis (which is horizontal in FIG. 4A, passing through the center ofterminal T1).

In various aspects of the invention, writing one bit of data requiresapplication of two overlapping pulsed currents: The first current flowsthrough the long metal strip 419 to induce the magnetization vector ofthe portion of the magnonic polarizer 417 (see FIG. 4B) that lies underone column of memory cells (see the dotted line 420 in FIG. 4A). Thedirection of this current determines whether 0 or 1 is subsequentlywritten. A transistor or diode, located in the CMOS substrate anduniquely associated with the selected cell, applies the second currentflow between terminals T1 and T2 to excite, by MST, the transientprecession of the moment vector of the free magnet 415, which culminatesin a switch between 0 and 1 information states. The recorded informationstate in free magnet 415 is subsequently read in the normal manner, byapplying a current pulse from terminal T2 to terminal T1 that is tooweak and brief for the switch. The resulting voltage differential acrossthe magnetic tunnel junction 414 senses the 0 or 1 information state.

In prior art, the torque due to transfer of spin momentum is driven by,and is proportional to, an electric current. In the here invented“magnonic” (or, equivalently, “thermagnonic”) spin transfer (MST), thetorque τ is excited by, and is proportional to, the flux of heat, notelectric current. Because heat power due to Joule heating is RI², thistorque τ is proportional to the square of the applied electric current(or voltage) that powers the heater. Due to a square functionrelationship of heat flux, the magnitude of current used to switch theinformation state in free magnet 415 is significantly lower (e.g. anorder of magnitude lower) than a switching current used in the priorart.

To compare the effectiveness of different spin-transfer torquemechanisms, one may define a numerical quantum torque yield εcharacterizing the special case of the spin moment of the free magnet(e.g. magnet 314 in FIG. 3B or magnet 415 in FIG. 4B) lying orthogonalto the pinned spin moment in magnonic polarizer (see magnet 318 in FIG.3B or magnet 417 in FIG. 4B). To illustrate the meaning of this yield,take first the prior-art spin-transfer torque created in acurrent-driven magnetic tunneling junction (MTJ). Let Δq be the numberof electron charges within a given time interval that tunnelquantum-mechanically through a tunnel barrier (which is included in somebut not all embodiments of a heater, e.g. tunnel barrier 315 in FIG. 3Bor tunnel barrier 411 in 4B). Next, let Δs be the spin momentumcomponent (in units of Planck constant divided by 2π) consequentlytransferred orthogonally to the total spin moment of the free magnet(e.g. magnet 314 in FIG. 3B or magnet 415 in FIG. 4B). The formulaε=Δs/Δq defines the quantum torque yield.

Consider first the quantum torque yield of an MTJ having a tunnelbarrier between ideal half-metallic ferromagnets. By definition,electrons having only one spin direction are free to tunnel to or fromany half-metallic ferromagnet. Suppose that one such electron tunnelsthrough the barrier, giving Δq=1. It transfers its entire spin momentum,contributing the amount Δs=½ to the torque if the moments areorthogonal. The resulting quantum yield is ε_(mtj)=½ according to thesaid definition of ε. The quantum torque yield for an MTJ of anycomposition cannot exceed this value ½ when using unbound (i.e. free)electrons having only one spin direction, e.g. using ideal half-metallicferromagnets.

A similar definition of quantum yield serves to characterizethermagnonic spin transfer in the schemes of FIG. 1A and FIG. 2A, asfollows. In several aspects of the invention, at the interface 191between the magnonic polarizer 112 and the normal-metal spacer 113 thereexists an atomic monolayer 112M composed of atoms to each of which arebound several 3d electrons. See FIG. 5A, wherein the ferrite 112F andmagnetic monolayer 112M together constitute the magnonic polarizer 112.Arrows in FIG. 5A, e.g. in regions labeled ferrite, normal metal, andfree magnet indicate directions of spin polarization. Optimally, eachatom of this magnetic monolayer 112M consists of an Mn or Fe nucleuswith the electron structure 4s3d⁵ outside a closed argon atomic shell,in certain aspects of the invention. These five 3d electrons, all boundto a single Mn or Fe nucleus, form a total spin quantum number S=5/2 andorbital quantum number L=0. Any value of L different from 0 is avoidedin some aspects of the invention, because it would engender effects ofthe spin-orbit interaction which would relax the spin polarization andweaken the spin-transfer effect. Superexchange coupling of the monolayer112M to the ferrite 112F creates a molecular field K(>0) acting on eachof these local spin moments.

Thus, the j-th (with j=1, 2, 3, . . . ) interfacial atom has energyE_(j)=−Km_(j) where m_(j) is the quantized spin component along the axisgiven by the spin moment of the ferrite 112F. It takes any one of thevalues m_(j)=−5/2, −3/2, −1/2, +1/2, +3/2, or +5/2. A transfer of spincomponent from the ferrite to the j-th interfacial moment occurswhenever thermal agitation of this superexchange interaction creates orannihilates a magnon and changes m_(j) by the amount Δm_(j)=±1.Subsequently, the same-site sd exchange interaction within the electronconfiguration of an atom of the monolayer 112M restores the initialvalue of m_(j), while transferring a spin component amounting toΔs=Δm_(j) to the non-magnetic spacer. In the formula Δm_(j)=±1, the sign(−) holds mostly whenever heat flows from the magnonic polarizer 112 tothe free magnet 114 (see FIG. 1A) and the upper sign (+) holds mostlywhenever heat flows in the opposite direction (see FIG. 2A). Thetransfer of heat energy from ferrite to spacer resulting from each suchsequence amounts to −KΔm_(j).

To illustrate the spin transfer process, assume that a spinel ferriteserving as item 112F in magnonic polarizer 112 (FIG. 5A) interfaces aspacer 113 of noble-metal (Cu, Ag, or Au) composition. Numerical valuesof K expected for interfacial magnetic atoms having electronconfiguration 4s3d⁵ are inferred from a survey of measured properties ofspinel ferrites. Consider the examples of ferrites with compositionsNiFe₂O₄ and MnFe₂O₄ having a (100) crystal plane for interface. Valuesof K estimated from openly published measured properties of ferrites[See, for example, Sections 4.3.1-3 of G. F. Dionne, Magnetic Oxides(Springer, New York, 2009)] appear in the second row of Table 1.

TABLE 1 assumed G_(Kap) estimated NiFe₂O₄ MnFe₂O₄ ↓ ↓ ↓ K (meV) = 14 8112 MWm⁻²K⁻¹ ε_(us)/V (V⁻¹) = 20 14  45 MWm⁻²K⁻¹ ε_(us)/V (V⁻¹) = 35 31

Now imagine some unspecified heater, having electrical resistance V/I,where I is electric current and V(>0) is the applied voltage. Jouleheating creates the steady heat flow F=IV. For example, an MgO tunnelbarrier 315 as in FIG. 3B or tunnel bather 411 in FIG. 4B may serve asthis heater 111.

Make the following estimate of ε for this case. Consider the hot magnonscreated by thermalization of the energy provided by one electron fallingthrough the potential V within this heater 315, 411, so that Δq=1. Thisenergy amounts to eV, where e=1.60×10⁻¹⁹ coulombs is the magnitude ofelectron charge. Upon entry into the metal on either side of thisbarrier 315, 411, the electron collides with unbound conductionelectrons already present. After a succession of such collisions, theexcess energy of the tunneled electron is shared, in the form of kineticenergy, by a large number of existing conduction electrons. When thisheat of amount eV (originating from one electron) flows across theinterface 191 (see FIG. 5A) from spacer 113 to ferrite 112F the transferof energy through each interfacial atom in monolayer 112M is ±K (withthe algebraic sign depending on direction of heat flow), according tothe above discussion. Therefore, the number of interfacial transitionsfrom the passage of a single electron amounts to |eV/K|. Since each suchtransition transfers one unit of spin momentum, the spin transfer due tothe passage of one electron must now amount to Δs=±eV/K. Taking ourabove definition (ε=Δs/Δq) of quantum torque yield together with thevalue Δq=1, we have the inherent quantum torque yield ε_(inh)=|eV/K|.

The examples of measured ferrite properties shown in Table 1 imply K≈14and 8 meV for the two ferrite compositions considered. Let us assume thereasonable applied voltage V=300 mV. The above derived inherent yields,corresponding to these two compositions, are ε_(inh)=|eV/K|=300/14≈21and 300/8≈37 are more than 40 times the ε_(mtj)=1/2 of an MTJ. Theessential insight is that each “packet” e|V| of Joule heat provided bythe passage of each electron through the heater 111 (FIG. 5A) is capableof the great number (e|V|/K) of spin transitions, each involving oneunit of spin momentum. Counterintuitively, the smaller the amount oftransferred energy K, then the greater is this inherent quantum yieldε_(inh) of torque.

In practice the full amount of inherent yield ε_(inh)=|eV/K| mentionedabove is not necessarily available if conduction electrons or phonons,instead of magnons, also carry some of the heat flow through themagnonic polarizer 112 without contributing to the torque. Specifying aferrite composition or other ferromagnetic insulator for the magnonsource 112 eliminates this threat from conduction electrons. However,the effect of phonons remains significant. The Kapitsa interfacialconductance G_(Kap) is the parameter relevant to this effect.Unfortunately, experimental data for G_(Kap) across a metal/ferritejunction is not available.

Several of the estimates given here assume low and high values ofG_(Kap), shown in the first column of Table 1, taken from a range ofmeasured G_(Kap) values reported for eleven junction compositions, notinvolving magnetism, in an open publication [R. J. Stoner and H. J.Maris, Phys. Rev. B48, 16373 (1993-II)]. Using the input data shown inTable 1, quantum theory predicts the useful quantum torque yield ε_(us)divided by voltage V applied, for example, to a heater such as thetunnel barrier appearing in FIGS. 3 and 4. The third and fourth rows ofTable 1 give predictions of ε_(us)/V, corresponding to these two assumedvalues of G_(Kap), calculated from theory for each of two electricallyinsulating spinel-ferrite film compositions: NiFe₂O₄ and MnFe₂O₄.Consider a typical nanoscopic spintronic device value of V=300 mV whichis too small to degrade an MgO tunnel barrier such as the barriers 315and 411 in FIGS. 3B and 4B. The predicted torque yields shown in Table 1range between ε_(us)=4.2 and 10.5, which are between 8.4 and 21 timeshigher than the ε_(mtj)≦1/2 known in current-driven STT using an MTJ.

A rigorous direct argument gives the sign of the in-plane component ofmagnonic spin-transfer torque τ acting on the free magnet. Consider thecase that heat flows from the magnonic polarizer 112 toward the freemagnet 114 as in FIG. 5A. Magnons, each bearing −1 spin component alongthe ferrite moment axis S_(frt,ζ) carry heat through the polarizer andannihilate at the polarizer/spacer interface.

By the law of continuity for electron spin component, the σ-component ofspin momentum transferred to the free magnet 114 by annihilating magnonsmust also be negative. It follows that the torque exerted on thefree-magnet moment S_(fm) in Case 1 (FIG. 5A) tends to align itoppositely to the ferrite moment (see FIGS. 1B and 1C). Reversing thedirection of heat flow, as in Case 2 (see FIGS. 2B and 2C), reversesthis direction. However, reversing the sign of applied voltage V in aJoule-effect heater does not change the torque direction.

The quantum derivation described above provides some key requirementsfor high useful yield of thermagnonic spin transfer in many aspects ofthe invention. One requirement is the provision of an interfacialmagnetic monolayer 112M (see FIG. 5A) with sufficiently large exchangecoupling K (>0) to the ferrite moment. A second requirement is a largeexchange-coupling coefficient J_(sd) of this monolayer 112M to theconduction electrons of the metal comprising the non-magnetic normallayer 113 together with the said monolayer 112M. This same-siteinteraction coefficient J_(sd) may have either algebraic sign. A thirdrequirement is to ensure that the interfacial heat flow between thespacer 113 and the phonon channel (and conduction-electron channel, ifpresent) within the polarizer 112, as measured by G_(Kap), issufficiently weak.

In FIG. 5B, the angle θ is the angle between spin moment vector Sfm ofthe free magnet 114 and the spin moment vector Sfrt of the ferrite 112F.This angle θ is changed by heat transfer across interface 191 betweenelectrically resistive magnet 112 and spacer 113 in FIG. 5A. This angleθ is either 0° or 180° in the absence of heat transfer across interface191. Note that the heat transfer across interface 192 is not relevant tothe discussion. In electrically resistive magnet 112 as a whole (i.e. inthe combination of ferrite 112F and monolayer 112M) each magnonsimultaneously carries an amount of heat energy and a unit of spin andso the flow of heat is unified with the flow of spin in resistive magnet112.

When a magnon is annihilated at interface 191 (and simultaneously anelectron in spacer 113 is reflected from interface 191), heat and spinfrom the annihilated magnon can flow in different directions in spacer113, e.g. as illustrated in FIG. 9A. Therefore, the flow of spin current(same as spin momentum current) through the spacer 113 is not coupled tothe flow of heat through the spacer 113. Therefore, the spin current(illustrated by an arrow labeled “SPIN” in FIG. 9A) originates from theinterface 191 and flows freely through the spacer 113 via an imbalanceof the spin pairing in the spacer 113. In several aspects of theinvention, the monolayer 112M (also called interfacial layer) includesmagnetic atoms (or ions) comprising locally bound electrons whose spinmoments conduct heat efficiently (with high thermal conductances) to theinterior spins of both the ferrite 112F and (in the opposite direction)to the spacer 113.

In FIGS. 5A and 5B, Σ is a spin moment vector of 3d electrons inmonolayer 112M e.g. electrons bound to Mn nucleus, and σ is a spinmoment vector of free electrons in spacer 113. σ arises from acombination of annihilation of magnons at interface 191 and reflectionof electrons in spacer 113 by free magnet 114. Hence, σ has thedirection of the spin moment vector Sfm of the free magnet 114 whilehaving a rate of change in the direction of the spin moment vector Sfrtof the ferrite 112F. In FIG. 5B, torque τ that arises due to spincurrent is oriented at 90° relative to the spin moment vector Sfm of thefree magnet 114.

In FIG. 5A Wnm is the thickness of spacer 113 e.g. at a minimum of 2 nm,which is the smallest dimension possible to prevent touching between 112and 114 despite roughness therebetween due to fabrication. Note that theeffect of spin transfer is diminished inversely relative to the extentWnm by which exceeds a maximum of 30 nm which is the mean free path ofan electron in spacer 113. In FIG. 5A, Wfm is the thickness of freemagnet 114, e.g. 5 nm.

Easy axis of ferrite 112F is horizontal to the right in FIG. 5A, i.e. inthe −Z direction. Easy axis is a unique axis along which energy of thespin moment vector, as a function of direction, is at a minimum. Inseveral memory cells in accordance with the invention, easy axis of freemagnet 114 is selected to be at a value θ₀ that is non-zero relative tothe easy axis of ferrite 112F. For example, θ₀ may be predetermined formemory cells to be 20° or 45°, depending on the aspect of the invention.Such fixed alignment (also the initial alignment) between th two easyaxes creates a non-zero initial torque and thus better ensures a switchof the free-magnet moment, on transfer of spin between the two magnets.In several oscillators in accordance with the invention, easy plane offree magnet 114 is selected to be orthogonal to the easy axis of ferrite112F. Easy plane is a plane wherein the spin moment vector has the sameenergy in all directions (in the easy plane), and the energy in thisplane is at a minimum relative to the spin moment vector's energy in alldirections in 3D space.

In some aspects of the invention, two methods are used for fabricatingan interfacial magnetic monolayer 112M satisfying these requirements:

-   -   A satisfactory interface may occur simply in the course of        depositing the ferrite. The word satisfactory here means that        the atoms of the interfacial magnetic element have the required        4s^(x)3d⁵ (where x is near 1) electronic    -   state capable of providing a strong same-site sd interaction.        The strength f_(sd)=0.5 eV, assumed in the above calculations,        is known for the dilute magnetic alloys Cu:Mn, Ag:Mn, and Au:Mn.    -   If the oxidation of the ferrite is sufficiently strong, it may        happen that the magnetic atoms in such a naturally occurring        interfacial monolayer 112M may not have the 4s electron        requisite for such a strong sd-exchange. In this case, one must        deposit approximately one atomic element of Mn or Fe without        oxygen between the depositions of the ferrite and the        noble-metal spacer element. For, in the fully metallic case, the        presence of the 4s electrons and the resulting value J_(sd)=0.5        eV are well established.

For the sake of simplicity of calculation, the description outlinedabove treats explicitly a single magnetic interfacial monolayer 112M, asindicated in FIG. 5A. The discussion shows that the useful quantum yieldincreases with the exchange interaction between a 4s electron and a 3delectron bound to the same nucleus (e.g. Mn or Fe). The coefficientJ_(sd) in the description measures the strength of the sd interaction.Every atom of a ferromagnetic metal such as Fe, Co, Ni or alloy of theseelements is subject to a strong sd interaction. It follows thatdeposition of more than one layer of metallic magnetic atoms between theferrite 112F and non-magnetic spacer increases the number of sdinteractions participating in the spin transfer and therefore prove toincrease the useful torque yield to a value closer to the intrinsicquantum torque yield. Hence in some aspects of the invention, polarizer112 includes multiple layers 112M of metallic magnetic atoms adjacent tointerface 191 with spacer 113.

A fundamental characteristic of magnonic spin transfer in some aspectsof the invention is that reversing the direction of the electric currentthat drives the heater 111 does not change the direction of the createdtorque. This characteristic does not pose a disadvantage for its use inan oscillator 300 (FIG. 3B) because this device needs only onedirection, whether convergent or divergent as described above, of spintransfer torque in order to function. Therefore, the advantage ofgreater efficiency, discussed above, of MST is apparent. Its use in manyaspects of the invention improves performance in some combination of 1)increase of oscillation frequency, 2) decrease of input power, and 3)increase of output. In view of the above discussion, in several aspectsof the invention the thermagnonic quantum torque yield ε_(us) exceedsthe corresponding quantity for a magnetic tunnel junction, and thiscondition implies a great spintronic advantage, particularly inapplication to MRAM (e.g. see 400 in FIG. 4B).

The spin-transfer torque available during a write operation in an MRAMnormally limits writing speed, freedom from write error, and scaling tohigher memory density. The electric-current output of one transistorserving each memory cell must provide this torque by some form of spintransfer. (The existing limit on electric current available from atransistor is often considered to be near 1 micro-Ampere per nanometerof lithographic-feature width.)

In many aspects of the invention, a spin transfer torque is proportionalto the yield ε which means that thermagnonic spin transfer in variousdevices in accordance with the invention provides a marked advantagewith respect to writing speed, freedom from write error, and scaling ofmemory density. In some aspects of the invention, the investigatedferrite compositions NiFe₂O₄ and MnFe₂O₄, discussed above, are preferredfor application of MST to MRAM.

For use in magnetic random-access memory 400, 450 (see FIG. 4B), thefact that the direction of magnonic spin-transfer torque does notreverse with that of driving current is a real disadvantage. Reversingthe polarizer-moment direction with an external field, as describedabove and illustrated in FIGS. 4A-4C (or as described in any prior art)overcomes this difficulty. Nevertheless, this work-around in theMST-MRAM device described above for some aspects of the inventionrequires enlarging the cell area in comparison with that of prior-artSTT-MRAM.

Moreover, the reliance on a single sign of MST-switching current I₂permitted by the said work-around by transistor-driven polarizerswitching current I₁ presents the incidental benefit, cited in certainSTT-MRAM prior art, of making possible cell selection usingcell-dedicated diodes instead of transistors in certain aspects of theinvention. Diodes are capable of greater current output than transistorsfor a given lithography scale, which fact also contributes to increasedwrite speed and/or error suppression using MST in several aspects of theinvention.

The total current required for different kinds of spin-transfer torque,including MST in many aspects of the invention, scales as a constant inmemory cells (see FIG. 4B) that rely for thermal stability on uniaxialbulk magnetic anisotropy. It follows that the improved spin-transferefficiency attainable with MST as described above permits scaling downfor more scaling generations in accordance with the invention, thanthose achievable with the current-driven spin transfer of prior art.

In some aspects of the invention, structures 110, 120 (FIGS. 1A-1C,2A-2C) are operated by performing one or more acts 601-603 illustratedin FIG. 6A. Depending on the aspect of the invention, an act 601 may beperformed, to change a direction of magnetization of polarizing magnet112 (FIGS. 1A, 2A), e.g. by passage of a current in a long strip locatedadjacent to the structure. Initially, the magnetization direction inresistive magnet 112 (to be used as a polarizing magnet) may point in arandom direction, immediately after a structure 110, 120 is fabricated.Therefore, an optional act 601 may be performed in some aspects of theinvention, to orient the magnetization moment of resistive magnet 112 ina predetermined direction, either at the factory (e.g. in case of anoscillator) or during normal operation (e.g. in case of a memory cell).

For example, FIG. 6B illustrates a structure in memory cell 410(described above in reference to FIG. 4B) wherein a spin moment vector610 is shown to be oriented along an easy axis of magnetic anisotropyenergy in the resistive magnet 417 which happens to be in the positive Xdirection in FIG. 6B. The magnetic moment vector (which is not shown inFIG. 6B) is oriented in the negative X direction in FIG. 6B.

Initially, the spin moment vector of the free magnet 114 (FIGS. 1A, 2A)may point in any random direction, although eventually it comes to restin the direction of the easy axis of anisotropy energy therein, which isillustrated by arrow 613 in the negative X-direction in FIG. 6B. At thisstage, orientation of the spin moment vector 613 in the negativeX-direction may be sensed (see branch 604 from act 601 to act 603 inFIG. 6A) by a sensor in memory cell 410 which is read as a data bit ofvalue 0. The two equilibrium directions (in the negative X direction andthe positive X direction) of the free-magnet moment 613 represent thedigital 0 and 1 states of the memory element (also called memory cell).

In an act 602 (FIG. 6A), heat is transferred between spacer 113 andpolarizing magnet 112 (FIGS. 1A, 2A) by generating heat in a heater(e.g. by passing a current through the heating element). For example, anarrow 612 in FIG. 6C shows the flow of heat from the diamagneticmaterial in short strip 416 to the resistive magnet 417, acrossinterface 191 (described above). During the heat transfer, many of theheated electrons (carrying left direction spin of spin moment vector 613in FIG. 6B) travel downward through spacer 416 to interface 191 as shownby arrow 614 in FIG. 6C. Each left spin electron denoted by arrow 614carries one-half of a fundamental spin unit with direction along thenegative x-axis. During collision and subsequent upward scattering ofthis electron from the interface 191, this spin becomes reversed,forming now an upward moving right-spin electron moving in the upwarddirection through spacer 416, as shown by arrow 615 (FIG. 6C). Thechange of one whole negative-x spin unit possessed by the scatteredelectron is conserved by the simultaneous creation of one magnoncarrying one whole unit of negative-x spin.

Note that the flow of electrons denoted by arrows 614 and 615 are equalin magnitude although opposite in spin direction, and hence there is nonet flow of electric charge through spacer 416. Instead, a current ofelectron spin (i.e. spin current) is formed between magnets 417 and 415via spacer 416, as a net result of the x-component of the right spinflowing upwards as per arrow 615 and the x-component of the left spinflowing downwards as per arrow 614, and the two spin flows 614, 615 whenadded algebraically, form the spin current through spacer 416.

In spacer 416, the spin current (which is the flow of x component ofspin) transfers the right spin from spin moment vector 610 in resistivemagnet 417 to free magnet 415 thereby make the spin moment vectortherein precess around the negative X-axis, represented by arrow 613 inFIG. 6C. In all that follows, the precession of vector 613 shown in FIG.6C will for the sake of brevity be described as resembling the spinningof a top. Those skilled in the art will understand that the motion ofvector 613 can be more complicated, without invalidating the inferencesmade in this discussion. The rate of precession of vector 613 is severalorders of magnitude lower than the rate of an individual electronrepeatedly reflecting between interfaces considered in the spin flows614, 615. While heat is applied, a cone angle formed by precession ofvector 613 increases, i.e. vector 613 tilts farther away from thenegative X-direction, as shown in FIG. 6D. Eventually, as heat isapplied continuously, vector 613 goes from precessing around thenegative X-axis, through 90° relative to the X-axis, and then toprecessing around the positive X-axis as shown in FIG. 6E. After vector613 has switched to precessing around the positive X-axis, act 602 (FIG.6A) is completed.

On completion of act 602, heat is no longer applied and the spin currentin spacer 416 disappears, as shown in FIG. 6F. On passage of time, thecone angle of precession of the spin moment in free magnet 415 reduces(in the absence of heat transfer), and eventually vector 613 is alignedagain along the easy axis in magnet 415 but this time in the positive Xdirection as shown in FIG. 6G. At this stage, orientation of the spinmoment vector 613 in the positive X-direction can be read (whenevernecessary) by the sensor in memory cell 410 (shown in FIG. 4B) as a databit of value 1. Note that as the spin moment vector 613 is aligned withthe easy axis in magnet 415, this data bit of value 1 remains unchangedeven when power to memory cell 410 is turned off, thereby to implement astatic RAM (SRAM) in some aspects of the invention. When power is turnedback on to memory cell 410, this data bit of value 1 may be read,whenever and as often as desired.

At this stage, as shown in FIG. 6G, spin moment vectors 610 and 613 inthe polarizing magnet 417 and the free magnet 415 respectively areparallel to one another, both pointing in the positive X direction, tothe right in FIG. 6G. In devices of the type illustrated in FIG. 4A-4C,at such a stage (when vectors 610 and 613 are parallel to one another)heat flow in the downward direction (in FIG. 6G) cannot change theorientation of vectors 610 and 613 relative to one another. Hence, inseveral aspects in accordance with the invention, the direction of thespin moment vector 610 is reversed (to make vector 610 point in thenegative X direction) by applying a magnetic field as discussed below.After such a reversal of spin moment vector 610, downward heat flowstores a data bit of value 1 in memory cell 410, as follows.

Specifically, as illustrated in FIG. 6H, in act 601 an electric current(formed of electrons) 616 is passed in the positive Y direction (intothe plane of the paper in FIG. 6H) in the normal manner, through a longstrip 419 that is conductive. Optionally, simultaneous electric currentsmay be passed symmetrically through one or more pairs of additional longstrip lines identical and parallel to 419, all of which lie underneathnearby rows of memory cells shown in FIG. 4C (or 1109 above as in FIG.11B). This stratagem may switch a greater area of 417 but beaccomplished with a lower current density, thus decreasing the damagingeffect of electromigration. Passage of current 616 generates a magneticfield shown by arrow 617 that changes the orientation of the spin momentvector 610 in the resistive magnet 417, as shown in FIG. 6H. After thespin moment vector 610 in resistive magnet 417 switches over, thecurrent 616 is turned off as illustrated in FIG. 6I, and vector 610eventually becomes oriented along the easy axis in resistive magnet 417,now pointing in the negative X direction.

Thereafter, in act 602 (see FIG. 6A) heat is again applied as shown byarrow 612 in FIG. 6J. On transfer of heat through interface 191 at thisstage, the spin moment vector 613 in the free magnet 415 startsprecessing around the positive X axis, in the above-described manner.Any time after the spin moment vector 613 in free magnet 415 beginsprecessing around the negative X axis (as shown in FIG. 6K), the heat612 is turned off and eventually vector 613 comes to rest in thenegative X axis. This orientation of the spin moment vector 613, in thenegative X-direction can be sensed (see act 603 in FIG. 6A) in memorycell 410 as a data bit of value 0, e.g. by measuring voltage for apreset current in the normal manner.

As per the above description of FIGS. 6B-6K, acts 601-603 are performedintermittently in some aspects of the invention. However, as will bereadily apparent to the skilled artisan in view of this disclosure, inmany aspects of the invention, either or both of acts 602 and 603 areperformed continuously for an oscillator 300 illustrated in FIGS. 3A-3C.Specifically, act 603 is performed continuously in an oscillator 300 soas to sense the time-dependent x-component of the spin moment vector inthe free magnet 314 (FIG. 3B). Act 602 may also be performedcontinuously, e.g. in an oscillator wherein the spin moment vector ismade divergent from the easy axis by the applied heat, which istherefore applied continuously to keep the spin moment vector inprecession. When operating such an oscillator 300, act 601 is notperformed in some aspects of the invention.

In some aspects of the invention, spin moment vectors 710 and 713 in thepolarizing magnet 717 and the free magnet 713 respectively are initiallyparallel to one another as illustrated in FIG. 7A, and when heat flowsupwards as illustrated by arrow 711, the spin moment vector 713 startsprecessing, due to a spin current through spacer 716 that arises fromannihilation of magnons at interface 191 in a manner similar to thatdescribed above. Upward heat flow 711 into a heat sink 712 originates ina heater 720 which includes a heating element 718 (such as aJoule-effect heater including an ohmic resistor to generate heat), and athermal barrier 719 which is supported on a substrate (which may beformed of, for example, silicon, or metal, or glass).

Although a heating element is different from other elements of astructure in several devices in accordance with the invention, inseveral aspects of the invention, a heating element 801 is included inor is formed by an electrically resistive polarizing magnet 112 (FIG.8A), or by a spacer 113 (FIG. 8B) or by an electrically conductive freemagnet 114 (FIG. 8C). Also, devices in accordance with the invention,may optionally include additional thermal barriers, such as acylindrical thermal barrier 802 (FIG. 8C) that surrounds theabove-described elements 112, 113 and 114. Note that in some aspects ofthe invention, thermal barrier 802 is formed by air or vacuum.

Air or vacuum separates an electrically resistive polarizing magnet 112(FIG. 9A) from an electrically conductive free magnet 114 whichtherefore do not contact one another in accordance with the invention.Instead, in devices of the type illustrated in FIG. 9A both magnets 114and 112 are in direct contact with a spacer 113. Hence, the term “directcontact” is used herein to mean there is nothing in between.

In many such devices, electrically resistive polarizing magnet 112 is indirect thermal contact with spacer 113 so that heat transfers freelyacross an interface 191 therebetween (FIG. 9A). Also in several suchdevices, electrically conductive free magnet 114 is in direct electricalcontact with spacer 113 so that electrical charge transfers freelyacross an interface 192 therebetween (FIG. 9A). Although interfaces 191and 192 are on the same side of spacer 113 (upper-most side in FIG. 9A),in other aspects of the invention interfaces 191 and 192 are on oppositesides of spacer 113 as illustrated in FIGS. 1B and 2B.

In several aspects of the invention, electrically resistive polarizingmagnet 112 includes multiple layers, such as layer 901 layer 902 as wellas a ferrite 903 as illustrated in FIG. 9B and interface 191 is at asurface of monolayer 901 in direct contact with spacer 113. As notedabove, in some implementations layers 901 and 902 are only one atom inthickness (in the vertical direction in FIG. 9B), and therefore layers901 and 902 are also referred to herein as atomic monolayers.Furthermore, in some aspects of the invention, heater 111 includes alight-emitting diode 111L which is spaced apart from magnonic polarizer112 by a distance S_(LED) as illustrated in FIG. 9C. Depending on theaspect of the invention, the value of S_(LED) may be a few nanometers,e.g. 2 nm or the distance can even be 0 nm (i.e. in direct contact withpolarizer 112).

As noted above, electron spin is transferred between an electricallyconductive material of a free magnet and an electrically resistivematerial of a pinned magnet, via a spacer (such as a metal) that hasfree electrons (i.e. unbound electrons or valence electrons notpermanently associated with any atom) to perform the spin transfer, inaccordance with the invention. In several aspects of the invention, aspacer 113 is formed by a single material as illustrated in FIG. 9A,while in other aspects of the invention spacer 113 includes multiplematerials. The multiple materials of a spacer 113 in accordance withthis invention can take several forms, depending on the aspect.

For example, spacer 113 in some aspects of the invention is formed bytwo materials 113A and 113B that are respectively in contact withpolarizing magnet 112 and free magnet 114 as illustrated in FIG. 9D. Inthis example, metallic bonds are formed at an interface 193 betweenmaterials 113A and 113B. The metallic bonds ensure presence of freeelectrons at interface 193 so that a spin current can flow easilytherethrough. As another example, spacer 113 in some embodiments of theinvention is formed by a material 113C and a number of impurities 113D .. . 113G embedded in material 113C as illustrated in FIG. 9E. In certainaspects of the invention, material 113C is diamagnetic while impurities113D . . . 113G are paramagnetic and in some such aspects thecombination 113 is either mildly diamagnetic or mildly paramagneticalthough any magnetism in spacer 113 is sufficiently small to allow afree flow of electrons to transfer spin therethrough, between magnets112 and 114.

In some aspects of the invention, although an oscillator 300 (FIG. 3B)includes stack 120 (FIG. 2A) in several aspects of the invention anotheroscillator 1000 (FIGS. 10A, 10B) includes stack 110 (FIG. 1A). Asillustrated in FIG. 10B, oscillator 1000 includes a silicon substrate1010 with a diffuse silicon resistor 1011 formed therein. Formed thereonare electrodes 1021, 1009 and 1022. Formed on electrode 1009 are thefollowing in sequence: electrically resistive magnet 1008, atomicmonolayer 1007, diamagnetic spacer 1006, free magnet 1005, tunnelbarrier 1004, electrically resistive magnet 1003, antiferromagnet 1002,electrode 1001.

In some aspects of the invention, although a memory cell 410 (FIG. 4B)includes a long strip 419 at the bottom (below resistive magnet 417), inseveral aspects of the invention a strip 1109 is located at the top(above electrode 1101) as illustrated in FIG. 11B. As illustrated inFIG. 11B, memory cell 1100 includes a silicon substrate 1110 with atransistor 1111 formed therein. Coupled to transistor 1111 is anelectrode 1108 that is located in a via hole through resistive magnet1107, and is coupled to a short electrode 1106 that lies on top ofresistive magnet 1107. Formed thereon are the following in sequence:magnetic tunnel sensor 1103 (which includes a free magnet, tunnelbarrier, pinned magnet #2, normal metal, pinned magnet #1,antiferromagnet) and formed thereon a thermal barrier 1102, and formedthereon electrode 1101. Elements 1101, 1102 and 1103 together constitutean elliptical pillar on which is located the long strip 1109.

In certain aspects of the invention, an integrated circuit containing amemory cell 1100 (see FIGS. 11A-11B) that uses Version #2 stack 120 (seeFIGS. 2A-2C) is fabricated as described below, in one or more of steps(A1)-(A9) either alone or in some combination with one another:

(A1). Begin with a conventional silicon wafer substrate such as commonfor CMOS integrated circuits, comprising digital circuitry for poweringand sensing data in a rectangular memory array comprising rows andcolumns of cells to hold data bits (of binary value 0 or 1). Suchcircuitry in many aspects of the invention includes a DC current supplycircuit to provide 100 microamperes, and a voltage sensor for sensingthe direction of magnetization (and hence the data stored in a cell).

(A2). Sputter or CVD 20 nm thick ferrite on much of the wafer, havingcomposition preferably at least one of: manganese ferrite MnFe₂O₄, mixedlithium-iron ferrite Li_(0.5)Fe_(2.5)O₄, yttrium-iron garnet Y₃Fe₅O₁₂.

(A3). Form studs for connecting each eventual short electrode 1106through the ferrite to an individual, previously formed, transistor foreach cell.

(A4). Sputter metal for short electrodes over the wafer and deposit thepillar materials shown in the front view of FIG. 11B in accordance withU.S. Pat. No. 7,411,817 granted to Nozieres et al. These comprise, insequence, in pillar 414 (see FIG. 11B) free magnet 415 (at the bottom ofpillar 414) having thickness 3 nm, tunnel barrier (not labeled in FIG.11B), pinned magnet 425, normal metal (e.g. formed of Ruthenium to avoidmagnetic interaction between two pinned magnets), pinned magnet 423,antiferromagnet (not labeled in FIG. 11B), thermal barrier 412, andelectrode 421.

(A5). Use a subtractive lithography method to define the pillar cylinderdown to the level of the free-magnet/short-electrode interface. Therectangular or elliptic pillar cross section has aspect ratio near 2:1.In several aspects of the invention, the dimension Ed (see FIG. 11A) is45 nm, based on integrated circuit (IC) technology using 45 nm linewidthlithography. In some aspects of the invention, Sw is 2 times Ed (i.e.2*Ed, or 2×Ed, wherein “*” and “x” both denote multiplication).

(A6). Use subtractive lithography again to define the lateral shapes ofthe short electrodes whose dimensions are the minimum permitted by theline width of the lithography technology. In some aspects of theinvention, Sw is 2 times Ed and the structure of the memory along thevertical in FIG. 4A is 4×Ed. The horizontal length of the short Stripelectrode 416 is 6×Ed and the horizontal period is 8×Ed.

(A7). Fill the spaces between said pillars and up to a predeterminedlevel above the pillars with SiO₂. Planarize said SiO₂ until the pillarelectrodes are exposed.

(A8). Print or otherwise deposit the long electrodes (also called “longstrips”) to connect with pillar electrodes 421 (FIG. 11B), eachextending over one complete column of memory cells (in the verticaldirection in FIG. 11A). In some aspects of the invention, strip 1109(FIG. 11B) has a width of 3*Sw, wherein Sw is described above. In suchaspects of the invention, a column of memory cells has a period(center-to-center distance of two memory cells) of 4*Ed. Therefore, thelength of strip 1109 exceeds 4*Ed*N, wherein N is the number of cells ina column of an array, e.g. N=128 cells.

(A9). Raise the temperature sufficiently to temporarily eliminateexchange coupling pinning the upper metallic magnet 423 (FIG. 11B) andapply a magnetic field of 200 Oe horizontally parallel to the majordiameter of the pillar 414. Then turn off the field after allowing thework to cool. This operation permanently pins the upper metallic magnet423 in a horizontal direction and magnet 425 in the opposite direction.

In some aspects of the invention, an integrated circuit containing anoscillator that uses Version #2 stack 120 (see FIGS. 3A-3C) isfabricated as described below, in one or more of steps (B1)-(B8) eitheralone or in some combination thereof:

(B1). Begin with a conventional silicon wafer substrate such as commonfor CMOS integrated circuits, comprising digital circuitry for poweringand detecting oscillation, formed using 45 nm linewidth. Depending onthe aspect of the invention, such circuitry includes a DC current supplyof 100 to 300 microamperes and voltage sensor for the 5-30 GHz outputgenerated across to-be-fabricated terminals T1 and T2.

(B2) Deposit the material for the pinned resistive magnet in two steps,along the lines of N. N. Shams et al, J. Appl. Phys. 97, 10K305 (2005).Use a region of the substrate beside that supporting the digitalcircuitry. First, sputter 20 nm of Pt. Second, sputter 30 nm of bariumferrite (BaFe₁₂O₁₉) with the substrate temperature held at 475° C. toresult in the crystallographic hexagonal axis oriented perpendicular tothe plane of the substrate.

(B3) Sputter the base electrode material composed of copper. Deposit thepillar materials shown in the front view of FIG. 3B in accordance withthe thicknesses and compositions of MRAM fabrication by U.S. Pat. No.7,411,817 (see above) These include, in sequence free magnet, tunnelbarrier, metallic magnet, antiferromagnet, thermal barrier, andelectrode.

(B4) Use a subtractive lithography method to define the pillar cylinderdown to the level of the free-magnet/base-electrode interface. In someaspects of the invention, the pillar diameter is between Dp=200 nm andDp=500 nm, depending on the desired output power of the oscillator 300(FIG. 3B).

(B5) Again use subtractive lithography to define the lateral dimensionsof the base electrode together with the pinned magnet. The dimensions ofthe base electrode and pinned magnet are 1.5Dp×2Dp (wherein Dp is thepillar diameter as noted above).

(B6) Solder the leads from the control circuitry to the terminals T1 andT2.

(B7) Apply an external magnetic field exceeding 3 kOe verticallyupwards, and then remove it, in order to leave the resistive magnet in aupward magnetized remanent state.

(B8) Raise the temperature sufficiently to temporarily eliminate theexchange field acting on the upper metallic magnet, and apply a magneticfield of 200 Oe horizontally. Then turn off the field after allowing thework to cool. This operation pins the upper metallic magnet in ahorizontal direction.

In some aspects of the invention, an integrated circuit containing anoscillator that uses Version #1 stack 110 (see FIGS. 10A-10B) isfabricated as described below, in one or more of steps (C1)-(C12) eitheralone or in some combination thereof:

(C1). Begin with a conventional silicon wafer substrate of the typecommonly used in normal CMOS integrated circuits, comprising integratedcircuitry for powering and detecting oscillation. Depending on theaspect of the invention, such circuitry includes one primary DC currentsupply of 0.1 to 1 milliamperes for input power to the heater and asecondary supply of 1 to 10 Volt DC for receiving the 5-30 GHz generatedoutput.

(C2). Form base diffusion and ion implant sufficiently to create anion-implanted resistor material 1011 (FIG. 10B) on the surface of thesubstrate 1010, having 5 kOhm per square resistance. Perform thisoperation in a region of the substrate 1010 aside from that containingthe said integrated circuitry.

(C3). To deposit the Pt material for forming electrodes 1009, 1021 and1022 and a hexaferrite precursor for the resistive magnet, sputter 20 nmof Pt.

(C4). Deposit the pinned resistive magnet material 1008 by the methoddescribed by N. N. Shams et al, in J. Appl. Phys. 97, 10K305 (2005)entitled “Magnetic properties of BaM/Pd—Pt double-layered thin filmdeposited at various substrate temperatures” and which is incorporatedby reference herein in its entirety. Sputter 30 nm of barium ferrite(BaFe₁₂O₁₉) 1008 with the substrate temperature held at 475 C to resultin the crystallographic hexagonal axis oriented perpendicular to theplane of the substrate. Enhance the MST effect by depositing an atomicmonolayer 1007 of Mn on said ferrite in the presence of oxygen.

(C5). Sputter the remaining pillar materials, above said spacer 1006,visible in the front view shown in FIG. 10B in accordance with thethicknesses and compositions of a conventional field sensor comprising amagnetic tunneling junction. These materials include, in sequence, afree magnet 1005, a tunnel barrier 1004, a metallic magnet 1003, anantiferromagnet 1002, and the pillar electrode 1001 (FIG. 10B).

(C6). Use a subtractive lithography method to define the pillar cylinderdown to the level of the pinned-magnet/Pt-precursor interface. Thepillar diameter will be between Dp=200 and Dp=500 nm in thickness,depending on desired output power.

(C7). Again use subtractive lithography with a different mask to definethe lateral dimensions of the three platinum electrodes 1021, 1022 and1009. Electrodes 1021 and 1022 have exemplary dimensions of 200 nm×100nm. Electrode 1009 has exemplary dimensions of 150 nm×100 nm.

(C8). Connect the leads from the said primary supply to Pt electrodes1021, 1022.

(C9). Connect the leads from the said secondary supply to the pillarelectrode 1001 and Pt electrode 1009.

(C10). Apply an external magnetic field exceeding 3 kOe verticallyupwards, and then remove it, in order to leave the resistive magnet in aupward magnetized remanent state.

(C11). Raise the temperature sufficiently to temporarily eliminate theexchange field and apply a magnetic field of 200 Oe horizontally. Thenturn off the field after allowing the work to cool. This action pins theuppermost metallic magnet 1003 in a horizontal direction.

Some aspects of the invention described briefly above are furtherdescribed in detail in the following article, which is incorporated byreference herein in its entirety: “Initiation of spin-transfer torque bythermal transport from magnons” by John C. Slonczewski, published onAug. 3, 2010, PHYSICAL REVIEW B 82, 054403 (2010).

In numerous devices in accordance with the here-invented thermagnonicSTT, a material that is used to form a polarizing magnet is selected forhaving a bulk electrical resistivity greater than ρ=1×10⁻³ Ω·cm,implying an areal electric conductance less than the order of 1×10¹³Ω⁻¹m⁻² for a 10 nm-thick polarizer. According to the Wiedemann-Franz law,this value in turn implies a free-electronic areal thermal conductanceamounting to the order of 1×10² MW/m²K which is comparable to typicalvalues of interfacial Kapitza conductance transported by phonons. Thusthis electrical resistivity greater than ρ=1×10⁻³ Ω·cm guarantees thatthe heat flow wasted by electron movements is smaller than the unavoidedwaste arising from phonons and approximately accounted for elsewhere inthis analysis.

Moreover, this bound on areal resistance RA>1×10¹³ Ωm² exceeds by twoorders of magnitude the resistance typical for the interface between aferromagnetic metal and a dielectric metal suitable for all-metallicthermoelectric STT. Consequently, the thermoelectric torque estimate ofHatami et al for thermally driven STT in an all-metallic trilayerstructure would be correspondingly diminished and thus appears to beinsufficient to switch the data state of a memory element. See Hatami etal. article entitled “Thermal Spin-Transfer Torque in MagnetoelectronicDevices” PHYSICAL REVIEW LETTERS 99, 066603 (2007) which is incorporatedby reference herein in its entirety.

To summarize, if this >1×10⁻³ Ω·cm bound on resistivity is satisfied, amajority of the spin transfer through the polarizing magnet is done bymagnons rather than electrons and its strength is estimated properly asdiscussed above, for various devices in accordance with this invention.Examination of published data on resistivities of ferrimagnetic oxidesreveals that nearly all compositions satisfy this bound, and hence arecontemplated by the inventor for use in various devices in accordancewith this invention. A borderline exception is Fe₃O₄, whose resistivityis about 1×10⁻² Ω·cm. Other possible exceptions found in the literatureare compositions doped with certain non-magnetic cations on purpose toincrease the conductivity for reasons not connected with STT. Examplesof satisfactory material compositions for a fixed magnet in numerousaspects of the invention exceed the just-described resistivity bound of1×10⁻² Ω·cm by at least 2 orders of magnitude. In some aspects of theinvention, any ion in a ferrimagnetic oxide which gives up an electronto an oxygen atom is referred to as a magnetic ion, and in severaldevices the electrically resistive material comprises a cubiccrystalline ferrimagnetic oxide and an outer electron shell of eachmagnetic ion in said cubic crystalline ferrimagnetic oxide has five 3delectrons or seven 4f electrons.

Through any solid possessing spontaneous magnetization:

-   -   Electric current is transported only by movements of free        electrons (if any are present).    -   Spin current is transported by movements of: thermal magnons        (always present) and free electrons (if any are present).    -   Heat is transported by free electrons (if any are present),        thermal phonons (quantized lattice vibrations, always present),        and thermal magnons (always present).

Hatami et al teach thermoelectric spin-transfer torque (STT) in anall-metallic (i.e. having free electrons throughout) trilayer systemcomprised of a polarizing magnet, a normal (i.e. dielectric) spacer, anda free magnet. They appear to consider heat-driven flow of spin that istransported only by movements of electrons. All transport by magnons andphonons appears to be ignored. On this basis, Hatami et al appear toteach (see their equation 7) that the transferred spin torque is simplyproportional to the mean electric conductance G of the system.

Numerous devices in accordance with the present invention ofthermagnonic STT differ from Hatami et al by regarding the polarizingmagnet as an insulator. In several aspects, the present inventionconsiders the magnon mechanism (which appears to be neglected by Hatamiet al) of transport through the polarizing magnet. In certain aspects,the present invention teaches the transport of spin by magnons throughthe polarizer to the polarizer/dielectric interface where thetransported spin converts to spin transport by electron movementsthrough the dielectric spacer, thence into the free magnet. But, in manyaspects of the present invention, G=0 in which case Hatami et al appearto teach that STT vanishes. Hence, Hatami et al appear to teach awayfrom numerous embodiments of the invention described herein.

Many devices in accordance with the invention include at least thesethree elements: an electric insulator possessing spontaneousmagnetization, an electric conductor possessing spontaneousmagnetization, and a non-magnetic metal positioned between these twoelements. Version #1 of such devices comprises in sequence theseelements: heater, magnonic polarizer, normal metal spacer, free magnet,and thermal disperser. Version #2 of such devices comprising in sequencethese elements: heater, free magnet, normal metal spacer, magnonicpolarizer, and thermal disperser. In some such devices, flow of heatfrom the heater creates torque on the moment of the free magnet. Inseveral such devices, the heater includes a thermal barrier to minimizewaste of heat.

Many such devices in accordance with the invention, include additionalthermally conducting materials inserted between said elements to make adevice whose function relies on spin-transfer torque. Several suchdevices include additional elements to make an oscillator, while othersuch devices includes additional elements to make cells of a magneticrandom access memory. In some such devices, the chemical binding betweena reservoir of magnons in the polarizer and the non-magnetic spacer issufficiently strong to insure easy flow of spin current.

In some such devices, the magnonic polarizer is composed of acubic-crystalline ferrimagnetic oxide in which the outer electron shellof each magnetic ion has five 3d electrons or seven 4f electrons. Inseveral such devices each of the local magnetic elements within themagnetic monolayer at the ferrite/normal-metal interface has a highelectron concentration in an unfilled atomic 4s shell. In certain suchdevices the oscillator is composed, in sequence, of a silicon CMOSsubstrate, pinned resistive magnet, base electrode with externalterminal, free magnet, tunnel barrier, pinned magnet, antiferromagnet,thermal barrier, and pillar electrode with external terminal. Innumerous such devices, the memory array is partly composed, in sequence,of a set of long parallel metal strips each lying underneath: one row ofa rectangular array of memory cells, a resistive magnetic layer havingvia holes, upon which rests said rectangular array of memory cells, eachof which is composed, in sequence, of a short metal strip with acylindrical electrode passing through said via hole, a free magnet, atunnel barrier, a pinned first magnet, a tunnel barrier, a second pinnedmagnet, an antiferromagnet, a thermal barrier, and an electrodesurmounted by an electric terminal.

Some devices in accordance with the invention use an electricallyresistive material which includes a crystalline material having onlyelectrons bound to atomic nuclei to constitute ions, the crystallinematerial lacking free electrons. Depending on the aspect of theinvention, the crystalline material may be either a single crystal orpoly-crystalline. In several such devices, the electrically resistivematerial includes one or more atomic monolayer(s) in contact with thecrystalline material, each atomic monolayer having atomic nuclei withelectrons having unbalanced spin, and each of the atoms in the atomicmonolayer having a partially-filled 3d electron shell. Hence, in manydevices in accordance with the invention, an electrically resistivematerial comprises a cubic crystalline ferrimagnetic oxide or ahexagonal crystalline ferrimagnetic oxide.

In several aspects of the invention, a tunnel barrier 315 (FIG. 3B) islocated at least partially between an electrode 321 and electricallyconductive material 314, a spacer 317 is located between theelectrically conductive material 314 and the electrically resistivematerial 318 and a terminal T2 is coupled to the spacer 317 (by aconductive trace) to pass electric current therethrough (i.e. throughspacer 317) to the electrode 321.

In numerous aspects of the invention, a thermal barrier 412 (FIG. 11B)is located at least partially between a first electrode 421 andelectrically conductive material 415, a second electrode 1108 is locatedat least partially in a via hole 1107V through the electricallyresistive material 1107, wherein the second electrode 1108 electricallycouples the spacer 1106 to a transistor 1111, and the spacer 1106 is inelectrical contact with the second electrode 1108, and a strip 1109multiple times longer than the spacer 1106, the strip 1109 (which passesthrough the plane of the paper in FIG. 11B) is oriented perpendicular toa longitudinal direction of the spacer 1106.

In several aspects of the invention, a tunnel barrier 1004 (FIG. 10B) islocated at least partially between a first electrode 1001 andelectrically conductive material 1005, wherein the spacer 1006 islocated between the electrically conductive material 1005 and theelectrically resistive material 1008, a second electrode 1009 locatedbetween the electrically resistive material 1008 and a diffuse-siliconresistor material 1011 so as to pass heat therethrough, and additionalelectrodes 1021, 1022 located in contact with the diffuse-siliconresistor material 1011 so as to pass electric current therethrough to atleast the second electrode 1009.

The above description of the disclosed aspects, embodiments, devices,methods, etc. is provided to enable any person skilled in the art tomake or use the present disclosure. Various modifications andadaptations to these aspects embodiments, devices, methods, etc will bereadily apparent to those skilled in the art, and the generic principlesdefined herein may be applied to other aspects, embodiments, devices,methods, etc without departing from the spirit or scope of thedisclosure. For example, although a thermal diffuser and a heater areused in combination to perform heat transfer in some aspects of theinvention, the thermal diffuser may be replaced with a cooler (e.g. athermoelectric cooler) with or without a heater in combination thereof,to perform heat transfer as described herein. Various methodologiesdescribed herein may be implemented by various means depending upon theapplication.

1. An integrated circuit comprising: an electrically conductive materialpossessing spontaneous magnetization; an electrically resistive materialpossessing spontaneous magnetization; wherein the electrically resistivematerial has an electrical conductance lower than the electricalconductance of the electrically conductive material, by at leastmultiple orders of magnitude; wherein the electrical conductive materialand the electrically resistive material do not contact each other; and aspacer comprising a metal, said spacer comprising atomically unboundelectrons to transfer spin between the electrically resistive materialand the electrically conductive material; wherein the spacer is indirect contact with the electrically resistive material.
 2. Theintegrated circuit of claim 1 wherein: the electrically resistivematerial comprises a crystalline material having only electrons bound toatomic nuclei to constitute ions, the crystalline material lacking freeelectrons.
 3. The integrated circuit of claim 2 wherein: theelectrically resistive material further comprises an atomic monolayer incontact with the crystalline material, the atomic monolayer comprisingatomic nuclei with electrons having unbalanced spin, and each of aplurality of atoms in the atomic monolayer has a partially-filled 3delectron shell.
 4. The integrated circuit of claim 1 further comprising:a heater located in thermal contact with at least one of theelectrically resistive material, the electrically conductive material,and the spacer, the heater being so positioned as to cause an externallysourced flow of heat at least between the spacer and the electricallyresistive material; a heat sink having greater thermal conductance thanthe electrically resistive material; wherein said heat sink is locatedin thermal contact with at least another of the electrically resistivematerial, the electrically conductive material and the spacer.
 5. Theintegrated circuit of claim 4 wherein: the heat sink comprises a siliconsubstrate; and the heater comprises at least one of (a) tunnel barrierand (b) Joule-effect heater.
 6. The integrated circuit of claim 4further comprising: a thermal barrier having smaller thermal conductancethan the electrically resistive material; wherein the heater is inthermal contact with each of the thermal barrier and the electricallyconductive material.
 7. The integrated circuit of claim 4 wherein: theelectrically resistive material is located at least partially between,and in direct contact with, each of the spacer and the heater.
 8. Theintegrated circuit of claim 1 wherein: said metal is copper.
 9. Theintegrated circuit of claim 1 wherein: the electrically resistivematerial comprises a cubic crystalline ferrimagnetic oxide.
 10. Theintegrated circuit of claim 1 wherein any ion in a ferrimagnetic oxidewhich gives up an electron to an oxygen atom is hereinafter referred toas a magnetic ion, wherein: the electrically resistive materialcomprises a cubic crystalline ferrimagnetic oxide and an outer electronshell of each magnetic ion in said cubic crystalline ferrimagnetic oxidehas five 3d electrons or seven 4f electrons.
 11. The integrated circuitof claim 1 wherein: the electrically resistive material comprisesmeghamite γFe₂O₃.
 12. The integrated circuit of claim 1 furthercomprising: a tunnel barrier located at least partially between anelectrode and the electrically conductive material; wherein the spaceris located between the electrically conductive material and theelectrically resistive material; a terminal is coupled to the spacer topass electric current therethrough to the electrode.
 13. The integratedcircuit of claim 1 further comprising: a thermal barrier located atleast partially between a first electrode and a heater; a secondelectrode located at least partially in a via hole through theelectrically resistive material; wherein the electrically resistivematerial is continuous, aside from a plurality of via holes of a memoryarray including at least one via hole for each cell in the memory array;wherein the second electrode electrically couples the spacer to atransistor and the spacer is in electrical contact with the secondelectrode; and a strip multiple times longer than the spacer.
 14. Theintegrated circuit of claim 1 further comprising: a tunnel barrierlocated at least partially between a first electrode and theelectrically conductive material; wherein the spacer is located betweenthe electrically conductive material and the electrically resistivematerial; a second electrode located between the electrically resistivematerial and a diffuse-silicon resistor material so as to pass heattherethrough; and an additional electrode located in contact with thediffuse-silicon resistor material so as to pass electric currenttherethrough to at least the second electrode.
 15. A method comprising:using heat flow through a structure to change a first direction ofmagnetization of an electrically conductive magnet to a second directionof magnetization; wherein the structure comprises said electricallyconductive magnet separated from an electrically resistive magnet by ametal spacer; wherein a majority of the heat that flows between at leastthe metal spacer and the electrically resistive magnet flows without anexternally-added electrical current flowing therebetween; and sensingsaid second direction of magnetization of the electrically conductivemagnet.
 16. The method of claim 15 wherein said using heat flowcomprises: generating said heat by passing current through a heater inthe structure; wherein said heater is comprised in or in direct thermalcontact with at least one of the electrically conductive magnet, themetal spacer, and the electrically resistive magnet.
 17. The method ofclaim 15 wherein: said sensing comprises using a magnetic tunnelingjunction.
 18. The method of claim 15 further comprising: passingsimultaneous electrical currents through a plurality of electricallyconductive strips parallel to one another and located adjacent to theelectrically resistive magnet in a memory cell, so as to generate amagnetic field that changes a direction of magnetization of theelectrically resistive magnet, each strip extending over a column ofmemory cells.
 19. The method of claim 15 wherein: said second directionof magnetization of the electrically conductive magnet sensed by saidsensing is different from another direction of magnetization of theelectrically resistive magnet prior to said using heat flow.
 20. Themethod of claim 15 wherein: the heat flows continuously to keep a spinmoment vector of the electrically conductive magnet in precession in anoscillator.
 21. The integrated circuit of claim 1 further comprising: amagnetic tunneling sensor located between the electrically conductivematerial and an electrode; wherein the magnetic tunneling sensorcomprises a tunnel barrier located between said electrically conductivematerial and another electrically conductive material.
 22. Theintegrated circuit of claim 1 wherein: the electrically resistivematerial comprises an atomic monolayer in said direct contact with thespacer.
 23. The integrated circuit of claim 22 wherein: the atomicmonolayer comprises atomic nuclei with electrons having unbalanced spin.24. The integrated circuit of claim 22 wherein: each of a plurality ofatoms in the atomic monolayer has a partially-filled 3d electron shell.25. The integrated circuit of claim 1 further comprising: a heaterpositioned to cause an externally sourced flow of heat at least betweenthe spacer and the electrically resistive material.
 26. The integratedcircuit of claim 25 wherein: the electrically resistive material islocated in direct contact with the heater.
 27. The integrated circuit ofclaim 1 further comprising: a heater located in thermal contact with theelectrically resistive material.
 28. The integrated circuit of claim 1further comprising: a heater located in thermal contact with theelectrically conductive material.
 29. The integrated circuit of claim 1further comprising: a heater located in thermal contact with the spacer.30. The integrated circuit of claim 1 further comprising: a heatercomprising a tunnel barrier.
 31. The integrated circuit of claim 1further comprising: a heat sink having greater thermal conductance thanthe electrically resistive material.
 32. The integrated circuit of claim1 further comprising: a heat sink located in thermal contact with theelectrically conductive material.
 33. The integrated circuit of claim 1further comprising: a heat sink located in thermal contact with theelectrically resistive material.
 34. The integrated circuit of claim 1further comprising: a thermal barrier having smaller thermal conductancethan the electrically resistive material.
 35. The integrated circuit ofclaim 25 wherein: said metal is silver.
 36. The integrated circuit ofclaim 25 wherein: said metal is gold.
 37. The integrated circuit ofclaim 25 wherein: the electrically resistive material comprises ahexagonal crystalline ferrimagnetic oxide.
 38. The integrated circuit ofclaim 25 wherein: the electrically resistive material comprises a cubiccrystalline ferrimagnetic oxide.
 39. The integrated circuit of claim 25wherein any ion in a ferrimagnetic oxide which gives up an electron toan oxygen atom is hereinafter referred to as a magnetic ion, wherein:the electrically resistive material comprises a cubic crystallineferrimagnetic oxide and an outer electron shell of each magnetic ion insaid cubic crystalline ferrimagnetic oxide has five 3d electrons. 40.The integrated circuit of claim 25 wherein: the electrically resistivematerial comprises nickel ferrite NiFe₂O₄.
 41. The integrated circuit ofclaim 25 wherein: the electrically resistive material compriseslithium-iron ferrite Li_(0.5) Fe_(2.5)O₄.
 42. The integrated circuit ofclaim 25 wherein: the electrically resistive material comprisesmanganese-magnesium ferrite (Mn,Mg)Fe₂O₄.
 43. The integrated circuit ofclaim 25 wherein: the electrically resistive material compriseslithium-zinc ferrite (Li,Zn,Fe)₃O₄.
 44. The integrated circuit of claim25 wherein: the electrically resistive material comprises yttrium irongarnet Y₃Fe₅O₁₂.
 45. The integrated circuit of claim 25 wherein: theelectrically resistive material comprises yttrium-gadolimium iron garnet(Y,Gd)₃Fe₅O₁₂.
 46. The integrated circuit of claim 25 wherein: theelectrically resistive material comprises barium hexaferrite BaFe₁₂O₁₉.47. The integrated circuit of claim 25 wherein: the electricallyresistive material comprises barium gadolinium-iron hexaferrite Ba(Gd,Fe)₁₂O₁₉.
 48. The integrated circuit of claim 25 wherein: theelectrically resistive material comprises barium aluminum-ironhexaferrite Ba(Fe, Al)₁₂O₁₉.
 49. The integrated circuit of claim 1further comprising: a tunnel barrier located at least partially betweenan electrode and the electrically conductive material.
 50. Theintegrated circuit of claim 1 further comprising: a terminal coupled tothe spacer to pass electric current therethrough.
 51. The integratedcircuit of claim 1 further comprising: a thermal barrier located atleast partially between an electrode and a heater.
 52. The integratedcircuit of claim 1 further comprising: a second electrode located atleast partially in a via hole through the electrically resistivematerial.
 53. The integrated circuit of claim 1 further comprising: astrip multiple times longer than the spacer.
 54. The integrated circuitof claim 1 further comprising: a tunnel barrier located at leastpartially between an electrode and the electrically conductive material.55. The integrated circuit of claim 1 further comprising: an electrodelocated between the electrically resistive material and adiffuse-silicon resistor material.
 56. The integrated circuit of claim55 further comprising: an additional electrode located in contact withthe diffuse-silicon resistor material.
 57. The method of claim 15further comprising: generating said heat by passing current through aheater in the structure.
 58. The method of claim 57 wherein: said heateris comprised in the electrically conductive magnet.
 59. The method ofclaim 57 wherein: said heater is comprised in the electrically resistivemagnet.
 60. The method of claim 57 wherein: said heater is comprised inthe metal spacer.
 61. The method of claim 15 further comprising: passingsimultaneous electrical currents through at least one electricallyconductive strip located adjacent to the electrically resistive magnet.